Skipstar, do you have the time & inclination to become Site Rep? You could put some ads up in the local free paper and get those empty plots filled.
Perhaps the council has a waiting list for these plots? Find out who the Allotment Officer is and badger them to get the plots filled ~ there must be loads of people who will take them on, whatever state they're in.

We have a couple of unemployed but swarthy men on our site, and they are more than happy to chop up brambles all day in exchange for a couple of pints. They have cleared a dozen overgrown plots on our site, by hand. Dug trees up and everything.

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~ What do I think of Western civilisation? I think it would be a very good idea ~ Gandhi