I agree wholeheartedly re. the Sainsburys Visa deal.
Instead of paying off the CC in full , I simply pay the monthly minimum (to keep the 0% deal valid) and subscribe fully to both the Abbey & Halifax 7% monthly savers. Subscription to both these monthly saver accounts has been timed for them to mature in advance of the anniversary of the 0% deal on my Sainburys card so the savings built up can be used pay off the card balance up to £9K and I get to keep all the interest.
The 7% interest is worth 5.46% at basic rate and 4.2% at higher rate tax. Assuming spending is equally spread throughout the year this equates to an effective rate of payback of 2.73% or 2.1% respectively on the entire credit card balance up to £9000. In reality, my spend is likley to be front-loaded somewhat due to Xmas so the effective rate of payback on my spending is likely to be higher than this.
Of course, an ISA would be a better savings vehicle for higher rate taxpayers but I've already subscribed for this year.
RM