If I can offer any advice about objecting to a planning application to build on allotments I will. At the Victoria Jubilee site (finally opened on 21 June 10) we 'lost' to the developer but we did ensure that instead of building on the whole VJA site as originally proposed the developer built rather attractive houses on one third of it and reluctantly and after long long delays made over the remaining land for allotments and playing fields - the latter still not ready though. The key thing is to ensure people know you have a robust local demand for plots on the site - ideally more than the number of plots available. This may mean some leg work.
The thing today is that the government is sending out strong messages to local planning authorities about encouraging urban food growing. Allotments compared to how they were even a decade ago are popular with government. There are few council's, certainly not Birmingham, unaware of the growing waiting lists for them. That said there's also great pressure to build more homes so it's vital that you prove there's a demand for all the plots on this site. If the site is less than perfect - as happens with neglect (and developers rely on this, to claim there;'s no demand) - demand will be lower, so in measuring demand make sure you ask people "would you like a plot on this site if...and then mention things like loos and kitchen in a communal shed, shared composting, parking space, security of the site and secure tool storage, water supply, and a good committee...
In this situation of being caught between a rock and a hard place the developer must be pressed to look for a brownfield site - and not build on allotments for which there is a waiting list. Contact me if you'd like: 0121 554 9794 and s.j.baddeley@bham.ac.uk.