Hi Mandy
I have recently had a greenhouse so I looked into this. The info I received from the supplier was not to have a solid base. The reason being that water will tend to pool on the floor making it slippy. They suggested using large heavy paving slabs and sitting them on sharp sand but not concreting them in. That way the water seeps through the gaps. Anyway, I did this and I am really pleased with the results. Also I managed to pick up the slabs for £25 so a good result!
Jools