Thanks for all your feedback! It seems that it may be a lot of hassle as our garden gets a fair bit of wind and all the locations I can put it aren't entirely sheltered.
so after reading all your replies I have seen a secondhand 6x4 polycarb steel frame greenhouse for sale and going to pick it up Saturday. The zip ups ones seemed to all be around £30-£40 and this secondhand polycarb I'm picking up is costing me £40... It seemed a reasonable enough gamble, plus it's a lot heavier. The place where I'm planning to put it is to slot over a raised bed, do you think this will work? Sorry for changing the subject!
Here's a picture of my proposed sight:
Placing it over here will mean the raised bed frame is on the inside so the soil won't touch the greenhouse. I will put a slab down in the middle where I've trodden it down. Also, the front two length of wood will come off so I'm not stepping over them to get in!
It will still need a solid base of some sort and be screwed to it, poly ones blow over as well. Maybe cement some bricks in around the edge to bolt it down on.