There are a few sites aimed specifically at allotment holders. The Wirral Federation of Allotments Societies has a comprehensive summary of allotments on the Wirral, where all the sites are, available plots, details of next meeting and much more. The Government Communities Department site covers the legal stuff; health and safety, what can and can't be done on a plot etc. It also has some useful links, including one to The National Society of Allotment and Leisure Gardeners Ltd. Another community site is Allotments UK which claims to be the UK's largest.
There are also two occurrences of Japanese Knotweed. The easiest solution here is to use an herbicide only available to licensed users, employing whom will cost the Council over £200.00 per plot!
For an excellent catalogue of weeds, and weed info in general, look at the
HDRA Organic Weed Management site. The Downs Garden Services site mentioned above also has a good section on weeds.