Here are some links to helpful organisations and forums.

Birmingham City Council Allotments the Council page on Allotments giving rents, rules etc.

Birmingham & Districts Allotment Council the council representing Birmingham Allotments.
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National Society of Allotment & Leisure Gardeners the main national body representing allotment holders.

National Vegetable Society a society aimed at show vegetable growers but with useful general tips on growing vegetables.

Garden Organic  the site for Ryton Organic Gardens with  a good range of common and unusual seeds.

Allotment4All a very active forum for allotment gardeners.