If you are going halves on the allotment then they should surely only have half the greenhouse? I would ask them to contain their stuff to their half and maybe suggest they use staging to get more into their space f they have that much stuff, however it doesn't seem fair if they are using your greenhouse when they hae a poly tunnel of their own why don't you fill their poly tunnel with your stuff and if they object tell them its becasue you have no pace in your greenhouse. Sounds like they are getting more than half of the allotment to me if they have the space the poly tunnel is on all to themselves AND use of the greenhouse. Or what about asking them to go halves with you on another greenhouse, ure they won't want to but it might get the messgae across that you need some space in the exisiting one