hello andy,
apart from building a permanent cold frame, you can also make 'tempporary' ones in the spring when you actually need them. people on our site do it all the time because our plots are quite small. all you need are a few planks of wood, some short timber 'pegs' (or metal, whatever is to hand to hammer into the ground and stabilise the planks to form the sides). arrange four planks so they form an area which you can cover with your plastic, then drive in the 'pegs' to stabilise, e voila :)
and when it gets warm, and you no longer need the frame for your seedlings, you take it all down and use the bed space as you would normally.
just an idea :)
svea