I've just stumbled across this, entirely by accident.
The Dept for Communities and Local Government are running a consultation on removing statutory burdens from local authorities, one of which is the statutory requirement to provide allotments.
The consultation ends today, Easter Monday. Did you know about it? I bet you didn't. Nice one, CLG. Run a consultation but don't let interested parties know you're running it, then you can claim that because there weren't any replies no-one is that interested and you can remove the statutory duty with impunity. Pah; rumbled that one, matey.
You've just about got time to write to them. And your MP.