Mandy,

My GH base is slabs. OH dug out the ground to near level, had to use bricks as a retaining wall at one end as our ground slopes away. Filled in with soil topped off with sand and laid the slabs as a base. Our GH is bolted on with special bolts bought from B&Q. We have had some horrendous gales but no movement of the greenhouse. Also used this in our last house on the GH and never had any trouble.

Hope this helps
Janice