Yeah, minefield isn't it.

BED A Spuds, then peas (which is NEXT year's crop planted early). Then next year you should not plant peas, or anything that is attacked by whatever attacks peas [dunno if that included Beans, sorry]

BED B - "currently peas, broad beans, french beans - the peas are dieing off, so i can pull them up now and plant cauliflower to overwinter - and in spring, that cauliflower is cut and replaced with ...... something that's not a brassica???" Definitely not a brassica, as I think all brassicas are able to be affected by the same club root type disease.

I would then plant lettuce or some other catch crop

BED C Your Spring cabbage etc. (which has basically overwintered having been planted after your Peas and Beans last year) is then free for catch cropping until you plant your Spuds (or carrots etc.) next year.

"should i stick to just one sowing each year??"

If you've got loads of space that's OK, of course, but I don't think most people have, and Veg. gardeners tend no to like to see any bare earth!

Here's a bit from "Grow your own vegetables" by Joy Larkcom (scanned, so probably full of typos) :

ROTATION
Traditionally the cornerstone of garden planning - and a key factor in deciding where to grow each vegetable - is the rotation system. This is the practice of grouping together closely related vegetables and growing them in a different bed, or different part of the garden, each year, generally over a three- or four-year cycle, for the following reasons:

Pest and disease control

The main reason for rotation is to prevent the build-up of serious soil pests and diseases. These attack botanically related vegetables, and when their 'host' plants, to use the scientific term, are continually grown in the same soil, they can build up to epidemic proportions. If, however, an unrelated crop is grown in that soil for a few years, their numbers decline - or they fail to build up in serious numbers in the first place. The phenomenon known as soil sickness, where yields gradually decline year after year, is generally caused by soil pests such as eelworm (nematodes) and soil-borne diseases such as clubroot and onion white rot. (Clubroot can persist for twenty years; onion white rot for eight.) Rotation is the best form of preventive medicine. Rotation should also be practised in greenhouses and polytunnels. Soil sickness frequently develops if tomatoes, and related crops such as peppers and aubergines, are grown in the same soil for three or more consecutive years.

Soil fertility

Leguminous crops such as peas and most beans contribute to soil fertility by releasing nitrogen into the soil when they are dug in. For this reason they are often grown before brasicas, which have high nitrogen requirements. Crops also vary in the level of nutrients they require, and the depths at which they extract them. Ringing the changes allows depleted soil nutrients to be replenished naturally.

Weed control

Plants with dense foliage and sprawling habit - potatoes and pumpkins are prime examples - largely prevent weed germination while they are in the ground, whereas onions and carrots form a poor canopy, and are susceptible to weed competition. There is far less of a problem where they follow potatoes or pumpkins. Potatoes are considered a good ‘cleaning’ crop, partly on account of their dense leafy canopy, partly because the earthing-up process exposes weed seeds and soil pests to birds. (Slugs and wireworm are also removed from the soil when the potatoes are lifted.)

PLANNING

The following are the most important plant groups far rotation purposes:

Brassica/crucifer (cabbage) family: broccoli, ruds sprouts, cabbage, calabrese, cauliflower, kale, kohl rabi, oriental greens (for example Chinese broccoli, Chinese cabbage, choy sum, komatsuna, mustards, pak choi, Senposai), radish, salad mustard, salad rape, salad rocket, Texsel greens, turnips, swedes. In terms of rotation, brassicas that are in the ground for several months, such as cauliflower and winter cabbage, are far more significant than quick-maturing crops such as radish and salad rocket.
Legume (pea and bean) family: beans (including broad, French and runner), peas, leguminous green manures includng field beans, tares and clover.

Solanaceae (potato) family: aubergines, Peppers, potatoes, tomatoes.

Allum (onion) family: garlic, leeks, onions, shallots.

Umbelliferae family: carrots, celeriac, celery, parsley, parsnip.

In practice, the onion family are often grouped with the legumes, while root crops, including carrots and parsnips, are grouped with the Solanaceae (potato) family.

Many vegetables, for example courgettes, sweet corn, Swiss chard, spinach and most salads, pose little problem from the rotation point of view and can be fitted in wherever convenient, often as catch crops before or after the main plantings in each section. However, if lettuces have been attacked by root aphids, avoid planting in the same place for at least a year.

Here is a simple, three-year-cycle rotation plan:

Plot A Plot B Plot C
Year 1 Legumes Brassicas Potatoes
Year 2 Brassicas Potatoes Legumes
Year 3 Potatoes Legumes Brassicas

In this rotation the nitrogen-fixing root nodules of the legumes enrich the soil for the brassicas that follow. Members of the onion and umbelliferous families, salad plants and other vegetables are fitted in where possible.

Rotation practicalities

Flexibility of the bed system
The simple, traditional rotation plan above may be too inflexible for the modern gardener, implying as it does that one third of the garden is given over to each major group. Dividing a garden into six, seven or more narrow beds gives far more flexibility. It makes sense to treat legumes, brassicas and potatoes as the main groups (perhaps allocating more than one bed to them as needs dictate), but in addition allocating beds to alliums, umbelliferous crops and perhaps salads, and earmarking spare beds for ‘miscellaneous’ use.

For many years the bed allocation in my garden was roughly as foIlows:

Brassicas 3.5 to 4
Potatoes 4
Legumes 1.5 to 2
Alliums 2
Salads 3
Other roots 1
Miscellaneous 4

I try to leave at least a three-year gap before returning to brassicas, potatoes, legumes or alliums in any bed

Sequence

In practice, it doesn’t matter very much which group follows which. Arguments can be made in favour or against most sequences. The important factor is to ring the changes between the main rotation groups.

Time scale

The longer the rotation cycle the better: rotation over a five- or six-year cycle is highly recommended where feasible. While a three- to four-year cycle is sound practice, in reality a cycle of six or seven years is needed to rid a garden of pests like eelworm and some soil-borne diseases.

Rotation in very small gardens

In very small gardens effective rotation is notoriously difficult not least because the soil pests are to some extent mobile themselves. Moving a crop a few metres/yards in one direction will achieve little. Try to rotate but at least:

Avoid following a crop with another in the same botanical group.

Leave as long a time gap as possible before replanting with another of the same group.

Go in for diversity and intercropping, both of which slow down pest and disease attacks.

Watch for soil problems developing, and if they do, stop growing the affected vegetables.

Draw consolation from ‘the case for not rotating’, below!

The case for not rotating

As far as I know this was first expounded in the 1980s by Dr Bleasdale of the then Vegetable Research Station at Wellesbourne. It hinges on the facts that:

some soil pests and diseases are mobile, so rotation in limited areas is probably ineffective some serious pests and diseases survive many years in the soil, for example, potato cyst eelworm up to six, and clubroot and onion white rot possibly as long as twenty grouping together vegetables in the same botanical group can result in pests and diseases spreading more rapidly - a common example of this being potato blight spreading to nearby tomatoes.

A solution, especially in small gardens, is to grow vegetables in the same area until a problem arises. For example, onions can be grown on the same bed until there are signs of onion white rot developing. Then they can be moved to another bed with healthy soil. Where clubroot is a risk, a bed can be put aside for brassicas, and kept at a higher pH with more frequent liming, to counteract its impact.

Rotation summary
* Don’t lose sleep over rotation! Rotate as much as you can.
* At the very least avoid planting vegetables from the same group in the same area in consecutive seasons.
* Organize your garden in small beds to increase flexibility.

GROUPING FOR CONVENIENCE

Within the limitations of rotation, it is useful to group together vegetables that:
* mature at roughly the same time - this makes it easier to clear a patch of ground, dig it over thoroughly and possibly sow a green manure
* will be sown or planted at roughly the same time - this enables a cleared piece of ground to be put immediately to maximum use.

Typical groupings could be:
* spring-sown salad crops such as spring onions, early carrots, lettuce and cut-and-come-again salad seedlings
* crops which overwinter in the ground, such as leeks, celeriac, kohl rabi, Brussels sprouts and kales
* half-hardy summer vegetables, such as courgettes, tomatoes, sweet corn and peppers.

Herbs
It makes sense to plant frequently used culinary herbs as near to the kitchen as possible, or along the edges of beds so that they are easily accessible in bad weather.

Perennial vegetables
Perennial vegetables such as asparagus and rhubarb are best planted in beds or areas set aside for them, or at the extremities ofvegetable beds. They would not normally be included in rotation plans.