I don't use the greenhouses/polytunnel for extending the season - simply because the garden is surround by tall trees and hedge, as well as low lying (one of my boundaries used to be a small stream, now filled in but still gets damp during wet weather and especially winter) This means the garden gets cooler and damp than in the open as well as no sun at all in mid winter.
I do however, take advantage of raised temperatures during the summer period for growing warm loving crops. Last year (my first year - and probably the warm July weather helped too) I was very successful with chillies see this
link to my diary. This year the melon trial was a bust - mainly because pollinating was a problem (my fault really - I had hoped pollinating insects would be attracted inside with the door wide open as I just did not have the time or patience to do it myself. Chillies, peppers and tomatoes are easier because you just gently jiggle the flower heads as you water the plants) I have decided to try chillies again next year (I've got some new exciting seeds which I'm eager to grow)