What Ceres said.
If you think a councillors has behaved wrongly you can make a complaint to the standards committee (at least I think you still can, they're being abolished). It's almost certainly an interest, though whether it's a prejudicial interest is another matter. The councillor should declare their interest at the start of the meeting but I'm guessing that unless the issue affects them specifically rather than just generally it won't be a conflict of interest and there will be no obligation for them to withdraw.
There was a time when councils were actually required by the allotments acts to co-opt allotmenteers onto an allotments committee. The requirement has been repealed but it could still be done.
Your real complain is that you don't share your councillor's views. It's by no means unusual for councillors to express views that are technically unlawful, such as banning chickens on allotments, but if the cleark's on her toes she'll catch that, but there's noting to stop councillors saying stuff you don't agree with, that's just how representative democracy works.