I kind of agree with Snows, but then maybe a lot of the rubbish there wasn't in plain sight. Just go for it and see what they say, also take pics so they can see what you have done and still have to do then they'll know you mean business. Also, as there were various hazardous things in it which shouldn't have been left on the plot by the previous tenant and/ or the allotment renters, you may well have a bit more of a case by mentioning it as well.