As soon as you have paid off your credit card - cancel them in writing, except for one or two you want to keep.
Get a free credit report here:
http://www.experian.co.uk/
Be aware that it will cost �5 to get your actual 'rating' - and I'm not sure it is worth it.
You will be shocked at the amount of information that is held about you.
When you have paid off your debts this should be reflected quite quickly on the reports - but the 'rating' may take some time to change. Each lender has its own criteria.