First thing, if you keep taking food out of the soil you have to put food
back. Otherwise eventually your harvest will decline drastically year on
year. I've seen it happen with "new" allotment gardeners. Even some
"Growmore" thrown on the cabbage patch will help as it also contains lime.

We get two loads of well rotted horse manure delivered every year and that
goes onto our potato patch, about 4 inches thick, for the coming year. The
next year that patch becomes the cabbage patch and receives two 25Kg bags
of lime (to ensure the correct pH). We use a 4 year rotation so after 4
years every patch will have received manure and lime. Hope that makes
sense.
We also use seaweed extract as a liquid feed and chicken muck pellets and
Growmore and Tomato feed too.

Whilst the pH isn't that important for most things, unless it's way
outside normal, it is worth getting it checked or getting a meter as some
things, and brassicas are one, prefer certain values. All brassicas prefer
a slightly alkaline soil (above pH 7) which is why a lot of gardeners lime
their cabbage patch. The more manure you use the more important it becomes
to check your pH.

You aren't the only one who had problems with onions this year, I've heard
others say cauliflowers are difficult so you aren't alone on that one
either, however I always suspect that the pH may be the problem. The usual
problem with carrots is germination, it's why we keep our own seed, so if
you got good germination and eventually a good crop ensure you use that
same variety again, it suits your soil/situation.

Have noted all that Bob, thank you.
We do have a regular free delivery from a riding stable which contains a
lot of what's needed but the base of it is wood chip/saw dust and I am
told that this is not too good as it takes a long time to rot.

Wally