another quick possibility-buy or make some raised beds. Very very easy but can explain if need be. Or buy a cheap wooden sandpit (have found them for £15 in past).
Stick it over the plastic if need be, else put a weed suppresser under (you can get these at b&q)
Fill with soil from garden centre. Then buy some plants-tomatoes and beans are really really easy.
Wave at oaf as s/he passes.
I do think you need to go on the offensive a little and do make sure you know what the regulations are on your allotment, and how they are going to be interpreted. Get a copy of the regulations and if you are in any doubt, go down to the CAB (half the volunteers there probably have an allotment).
The council will have to show you are in breach of these regulations to throw you off. There are very few standard regulations, and stuff like what you can grow (flowers? fruit?), height of trees, percentage defined as "cultivated" differ a lot even sometimes within borough.
Another thing to talk about with them is what their policy is on new plot holders, some councils have a policy of helping them, doing some clearing for them, etc etc.
Another thing to chuck at them is the fact that you are gardening organically, they are meant to be supporting that sort of thing.
If things get iffy, start putting as much as possibly in writing.