It depends what you're trying to achieve Kittykat. If you're just aiming for frost free this will work and give you several extra degrees of protection on a really frosty night. As a general growing method I think you might run into more problems than you get advantages.
I agree with DDL about the advantages of outdoor growing. I put my seedlings outdoors as much as possible on milder days, and then indoors at night until they get too big and have to go in the greenhouse. By that time it's April and I put a bit of overnight heating on for them, just about 6* to stop them getting checked. That way it's not too expensive. I use an electric fan heater. I find this the most controllable.

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From each according to his ability, to each according to his needs.