just to slightly contradict you lossehead - Bradford & Bingley and Northern Rock both do 130% and 120% mortgages respectively and both are respectable companies. That said, neither would be of any particular help in your scenario andy - they are total mortgages.
I would suggest talking to your existing provider and getting a 'further advance' - this may be on a variable rate though. If the 20K you are looking to pay off is predomiately credit cards, then anyone of the 'tv rip off' companies will still be vastly lower than your credit card repayments.
But always remember - your house is at risk if you do not keep up repayments blah blah blah!