I'd approach the Welbeck Estate to see if they want to side with you. If the original covenant is that the LAND must be used for allotments or public purpose, then selling the land for other use ( which is what would be happening) would contravene that covenant. How such money would be used is irrelevant, otherwise covenants wouldn't apply to land but the value of the land. The Welbeck Estate may well be prepared to object to the council's interpretation. Council solicitors tend to be very shaky on their law where allotments are concerned.
I'm in Bolsover several times a week and would be interested in finding out more if you want to meet up.