Thanks for the replies and suggestions! I tried to take some pics but they dont show the levels clearly so there is no point posting them. Just to clear a couple of points though: The land that belongs to the house isnt very big and its all concreted over. The old greenhouse didnt sit on its own pad, it sat on a 'flat bit' of that. The new one is much bigger so wont fit on it.
My latest idea is to put down a bead of cement and lay the frame into it. I'd then make sure the frame is level and let it set. I could then fill the centre at one go and be able to tamp down and level it using a plank across the tops of the frame sides. Anyone want to shoot me down in flames?