"i would sow more peas / beans etc in that same bed in the spring"

Same bed, but NOT following an earlier crop of the same type. Lettuce and some other odds and sods like that don't matter so much, its the things that get diseases like Club Root and Carrot fly / nematodes that crop rotation is designed to give less chance of carrying from one crop to the next - by moving the next crop to land that hasn't seen that crop of 3, 4 or more years (depending on the length of your rotation)

"if spuds are followed with peas / beans (legumes etc) then i shouldn't really be planting brassicas (cabbage) there?"

Well, that's fine, provided that piece of land doesn't see Spuds again for 2 more years and Brassicas for 3 more. However, it becomes harder to remember which bit of land had which crop last, so for simplicity the plot is divided into a number of zones, matching the number of years in the rotation.

Then the preparation of each zone can be appropriate to what it will grow - e.g.

Year 1 : Roots zone - just some fertiliser before planting

Year 2 : Others zone - Manure plus fertiliser before planting

Year 3 : Brassicas zone - lime if the PH is acidic plus fertiliser before planting

but the reality is that no one has exactly the same amount of space requirements for each of the three veg types, so the rules get broken! But keeping the same plant off a particular piece of land for as many years as possible is the goal.

Lots of people plant Runners in the same place every year ...