Bit late for onions, but certainly do-able. Make sure they're specific winter onions, which should be around in the garden centres now. Broad beans, something like Sutton, will over winter well and give you a head start in spring. See if your local garden centre has winter cabbage plants on sale, but FGS net the buggers or the pigeons will love you!
If you have anything like a cold frame (plastic sheeting and somthing to hold it up off the ground, salad-y stuff is quick and easy, will give the kids someting to see growing quickly.
I'd recommend "Grow your own Veg" by Joy Larkcom as a really good down to earth book. Tells you about each vegetable type, when to plant etc. She also gives ideal planting suggestions for different needs.
Get your potatoes ordered from a seed merchant (we use Marshalls) then you can set them chitting in egg boxes. Get lots of seed catalogues to fantasise about what you're going to plant next spring.
We tried green manure from Suffolk Herbs (or possbly Organic seed company) but the Hungarian Rye Grass was a pain in the a$$ to dig in.
If it's a neglected allotment, you'll probably benefit from loads of horse manure too.
Well done on getting so far in 6 weeks!!
