Hi Steve, Ive got 2 greenhouses and I put them up both the same as each other. The first thing I done was put the base of the GH together, so you know exactly where the GH is going to be sited. I then marked out , with a shovel line around the base, allowing 4ins each side, IE. 8ins wide. I then dug out a trench where the base is going to go according to these marks, about 4ins deep, but this could vary if the ground is sloping from one side to the other. Smash some hardcore up or stones etc and put it in the trench. If you are laying a membrane place this on top of the hardcore. Lay concrete breeze blocks on their sides, spacing out about 2 -3 ins apart. Level up from corner to corner, and set in concrete inside and out side of blocks. Make sure you place the base of GH on top of blocks before setting concrete, so it fits OK.
When this is setting you can put the GH up separately and loosely bolt together.
Go and have a cup of tea and get a good nights rest.
Lay the base of GH on the now set block base and fix the GH to it. Use minimum 2ins screws, raw plugs. You will need help with putting the GH up. Once fixed loosely drill the base into concrete blocks and fix securely, tighten up all the nuts for GH.
I will put a picture of mine on my page , so that you can see how I finished the inside .
Concrete blocks £1 - £2 each B&Q
Sand and cement , couple of bags sand, 1 cement, approx £10
It is really worth taking time to make sure you get it fixed down securely, its got to last.
Best of luck.

Steve, sounds good to me, often greenhouses are put on a solid base given no place at floor level for growing. If you can place the greenhouse north to south with the door south. Vents two west and one east. Hope this helps.