Successional Sowing

peppers/tomatoes/chillies, don't sow any more, they all ripen roughly the same time, in july/august/spetember/october, depending. If you want more tomatoes, when they're bigger, take out a side-shoot and plant that.
Spring onions sow every couple of weeks. Beetroot, you probably need to sow more than two because not every seed will grow on, and you need to watch for slugs and snails eating your seedlings.
PSB, each seed that grows will turn into one big plant. Not really grown it for me, so no idea when yours will crop, but it's more of a feb/march/april crop than a summer one.
Basiul, sow a few seeds per pot, and when they get to about 6/12 inches, pinch out the top leaves and it will become bushy rather than leggy. They like hot weather though, unless you're keeping it indoors, hold off on the sowing for now.
Courgettes, don't sow until 4 weeks before your last frost, or at the time of your last frost. They hate cold weather, and they will die if you put them outside and they get frosted.

With lettuce, you could try a cut and come again variety, and pick salad leaves rather than whoe lettuce?

I don't grow lettuces anymore, just containers of cut and come again, and pick it as I need it.

I reckon sowings every three weeks would see you right through summer.

I'm having a nightmare trying to continuously sow so I don't have a blank month. Is my problem that I don't have enough ground space to sow a continuous crop? For example, my lettuce bed will only hold 20 lettuces maximum. Right now I have about 40 in the greenhouse waiting to go out. Should I be ruthless and cut them down to 10, then start another 10 this week? If that's a good idea, should I do this for everything I have in the greenhouse? (40 Brussels Sprouts, 60 leeks, and 30 Cabbages). Because I have no room left in the GH, I can't start my parsnips. Thought I had this cracked.

Chef, use cut-and-come-again lettuces (looseleaf, not a hearting one) and just pick leaves as you need them: the plant will grow more

40 sprout plants, are you kidding?
Each one takes up a square metre near enough, same with cabbages... and you have to net them

sprouts, leeks and cabbages have no need to be in the gh anyway, get them outside.

Cheers Two Sheds Will get them out tomorrow. 1sqm means I have 35 plants spare all in individual pots LOL Guess i'm going to have a half full GH tomorrow now Thanks again.

Welcome

now your gh is empty you can take some of Minamoo's 90 spare tomato plants

Re the lettuce, if you don't want to just have cut and come again then you need to sow a few seeds every couple of weeks. I tend to sow 2 seeds in each of a 6 cell module every two weeks and then thin to the strongest. When they're a reasonable size I plant out and that does the two of us over the summer. At the moment I'm starting them in the cold greenhouse but later in the year they'll be still sown in modules (to avoid slugs) but on some staging outside.

Oh and with peas, I sow about 50 or 60 every 3 weeks to get a decent ongoing crop. Usually have 5 rows like this plus an earlier few in the polytunnel.

Thanks for all of the advice! I feel like I am finally getting somewhere in planning what I can do with the space I have.

Can you eat 40 lettuces in a 2 or 3 week period? I sow 18 lettuces every fortnight. 6 each of three different varieties (Cos, Iceberg and Webbs types). They come-ready at different times, and despite my good intentions of sowing every 2 weeks it doesn't happen! so we generally have enough

60 leeks I expect you will be able to eat (we grow more than that), but I only grow about a dozen Sprouts and a dozen or so Cabbage.

Depends what you like, of course, but I think you should plan based on the length of time the Harvest Season is (for Summer Cauliflowers they won't "stand", once ready, for more than a fortnight), and how many you can eat - so for Cauliflower don't grow more than you can eat / freeze in a fortnight, and sow some more every fortnight. Sprouts will stand all Winter (generally speaking) and Cabbages for a long time too.

Keep a record of dates : Sown, Germinated, Pricked out, Planted out (if grown in pots, otherwise just the date sown), and the First/Last harvest date, and the number of plants. Make a note of whether you like the flavour, and if there were too many / too few. Then next year you will be able to plan better, and grow more/fewer according to how it went the previous year.

If you raise plants in pots the Sow / Plant dates will tell you when the plot needs to be ready (so you can be raising plants that will go in immediately after another crop finishes)

The Germinate date will tell you when to look in the airing cupboard for signs of germination

The PrickOut date will tell you when you should sow in order to avoid seedlings needing to be pricked out when you go away for a weekend break

Or, sow a tray of them, and plant a couple out every couple of weeks. They'll stay little in the trays/modules

Teehee! I have erred on the side of caution and optimism with my courgette and squash seeds. I assumed that all the seeds will grow and only planted two seeds of each variety. If they don't come up I'll plant more. But at least I won't end up with hundreds of courgette plants!! lol!