Banana time!
OH & daughter are dismantling her futon. It's also going to the tip & I thought she was taking it with her...
OH came home demanding his camera tripod, or my Dad's which went to his tip in '11. Anyway we knew OH's old one was in 3rd bit of daughter's wardrobe, blocked in by the futon......can you see where this is going yet? 
Becks that link re Norfolk G/H looks good- more solid than a plastic, but less expensive than something bigger. Add an inexpensive coldframe- or better still make your own:
a la Geoff H- Bricks to make a base, say 2or3 high. Old spare double glazing window/polycarbonate sheet on top & weighted down. Ok not nec the safest option with Jess around, but she's old enough now to learn mustnt touch. A coldframe is warmer than outside, but cooler than any greenhouse, so a useful extra.
Right, back to shed, which isnt prooving as bad as I thought/expected. J.