That's never as easy as it sounds. You start off on an informal basis, because it seems unfriendly to have a contract between neighbours. Then a bit further down the line you start getting niggles - they don't think you're weeding often enough, they feel you're growing too many courgettes and not enough beans, you think they've taken more than their fair share of strawberries, they want to to use some of the ground you've lovingly manured for the past 3 years and and plant a magnolia on it. And it all ends in tears.

You may be paying a lot for an allotment, but part of the benefit is a clear contract, so everyone knows what is expected of them.

A quick Google suggests average rents for allotments are about £25. I don't know what they are in your area, but £25 is reasonable compared with your other costs, seeds, compost etc.