bicci, you might want to have a look at what's happening with Farm Terrace allotments in Watford.
Their plots are under threat (you can read up on exactly why on their website), but the allotment holders there are mounting both a vigorous and creative campaign to defend them.
One key move they are making is to apply to have the allotment site nominated as a Community Asset, by using recently introduced legislation contained in the Localism Act 2011. I'm not sure how far they've got to date (you can contact them through their website) but you can read all about it here (click on 'press release' at the top of the page for the bit about Community Assets):
http://www.farmterrace.btck.co.uk/If you use it, Farm Terrace is also on Twitter @SaveFarmTerrace =
https://twitter.com/SaveFarmTerraceI would think seriously about setting up a Twitter account and/or a Facebook page to bring people together and help spread the word. Social media is proving that there is power in numbers which can be used for all sorts of good things - including protecting our treasured allotments.