tigwig - you are right, I was told only recently that keeping taking advantage of zero percent does affect your credit rating, because people can see you only paying a minimum, just moving it round.
I don't think it's so much to do with being self-employed (although you'd have to evidence your income), but the fact you haven't got balances on your other cards goes against you - a lender has no way of knowing that you're not going to rush out tomorrow and use all your available balance.
The mortgage extension is your best bet - that's a longer term and the rates are generally less than you're paying on a CC - one of my MBNAs was 26.4% before I paid it off. Go see your bank, don't delay, if you are a reputable customer there, they'll help, and there are some good deals around on remortgaging at the moment.