At the Allotment Sites Representatives' Meeting on 31st January, David Corby (the Council Officer Representative) said that a proposal for the Council to offer a (charged) service to clear previously disused allotment plots was going to Committee.
I have e-mailed David to ask if this service is now offered and how anyone who is interested can get further information, but as yet I have had no reply.
Can anyone else help?

Not at the moment - I'll ask around

But if I know Mr. Corby then you'll probably get more blood out of a stone

I know it seems like a rather circuitous way to do it, but I would suggest getting in touch with Susan Hall - the relevant cabinet lead - directly or via your local councillor.  She's been helpful and responsive on other issues,

Rahul