Each councillor must make their own decision about whether they have a personal interest in a council decision. They can ask for advice from the clerk, or from the Standards Officer (or anybody else). They should also consider how the matter would be seen by an impartial citizen. Nobody has to power to do more than advise a councillor on whether they should participate in a decision.
However, each councillor is subject to the law, and in extreme cases could be liable to criminal prosecution. In less extreme cases, they can be reported to the Standards Officer, although there are no sanctions available.
If the council is subsidising the allotments (rather than simply balancing income and expenditure), then it would certainly appear that an allotment holding councillor would have a personal interest in decisions about the level of subsidy. They would not necessarily have a personal interest in every decision concerning allotments.
In the event that a council cannot make a decision because there is no quorum of disinterested councillors, the District Council has reserve powers so that the matter can be resolved. The District Council's legal officer should be consulted.