That's a point - what kind is yours?
I suppose you could grow any of them in Devon, though I'd still be worried about a winter like the last one.
You might want to put a lot of water as a heat sink inside the cover - it's vastly more efficient at keeping frost out.
I only risk Meyer's Lemon outside in London - it's the hardiest (mandarin cross), but the last two winters have really hammered it to the ground. I'm not yet sure it will survive - but a clementine/mandarin in the same position escaped almost scot-free. The clem suffered about 25% damage in 2009/10 but the 3x2L water bottles I put in for December 2010 meant it survived the much worse 2010/11 without any problem.
Cheers.