Let's be very very clear about what stoozing is.
It is NOT spending money on credit cards.
Here's how it works. You borrow money on a 0% card, then you do not spend that money but instead put it in an account that pays interest.
You can then spend the interest earned on that money. Bear in mind you will be taxed on the interest.
You will need to pay back everything you have borrowed on the 0% credit card as soon as the 0% rate period ends. Often this is only 6 months. So you can't earn much interest in the meantime.
A alternative is to put the borrowed money in an account that is offset against your mortgage. This means that it notionally reduces the capital sum of your mortgage, and so means you pay less interest on the mortgage.
However, again, you will need to pay back the money when the 0% comes to an end, so then the capital sum of your mortgage will go back up.
Basically, it can work if you are (a) very disciplined and will not spend any of the borrowed money, only the interest it earns and (b) very organised so that you pay back the money borrowed as soon as the 0% is over.