Posted: 11/08/2012 at 22:40
I have a cold frame and have the same as Dovefromabove. A greenhouse to me is fully glazed, gets full sun and should never go below 0C. A cold frame is not about total sun and heat, it is about giving plants a boost out of the wind and a few C above outside temps. It certainly boosts plants like Hollyhock, Delphinium, Pumkin, Onions etc by a few weeks/perhaps a month at least compared to what they would be outside. From what you said you should go for a small coldframe as it will give you what you need. I start of everything in it - good if you buy annuals early but want larger plants in the border as it helps when pests attack to be bigger.
I also had one of those plastic 'greenhouses'. They do get warm in summer but they do not last and as you say the zip breaks and they fall over in the wind even with weights holding it down. I did find that Vine weevil likes the combination of warm interior and tasty seedlings . When the zip goes so does the heat and the cover can cost more than the 'greenhouse' itself.