If you pay rent to the Council I suppose they own the land.
I suggest (a) find out when they got it, and why ( it will be in their minute books way back). If they got it just for allotments it is Statutory allotment land.
(b) form a group - if there is no association on your plots, or it is inactive - and this can be quite informal to start with: the XYZ Allotment Group, for example. Ask to be represented on any Council discussions.
(c) contact the Allotment Regeneration Initiative (
www.farmgarden.org.uk/ari) and find masses of fact sheets and advice. Ask for or find your local mentor, see if he can advise you on getting funds for improvements, forming working parties etc.
(d) let us know how you get on! We can then have more ideas(!)