Hi Ian
As I understand it, you apply for a card that has a 0% interest rate on purchases or balance transfer. You then arrange for the full credit limit to be transferred to you bank account (there can be a fee for this upto around 3%). Use the money in a high interest account for the life of the deal that you have, and then pay this off before you start incurring interest.
Of course if you are charged a fee of 3% then you would need a pretty good high interest account to make this worthwhile.
I know MBNA allow this because I recently signed up for a credit card and they told me I could pay funds straight in to an account (at a 3% charge).
There may be more to it than that but this is how I understand it works.
The Big FC
The Big FC
Big is Beautiful