Thank you for your reply dave; however I'm trying to do this with Charge Rules and not shipping rules (because they are accumulative). I have three rules:
One that sets the P&P to £1.00 if the subtotal of the cart is between 2.99 - 15.99
One that sets the P&P to £2.00 if the subtotal of the cart is between 15.99 - 25.99
One that sets the P&P to £0.00 if the subtotal of the cart is above 25.99.
If I use shipping rules and the subtotal of the cart is 2.99 than the shipping is set to £1.00, however if the subtotal of the cart is changed to above 15.99 than the shipping is set to £3.00 i.e. £1.00 + £2.00.
The problem with the charge rules is that it uses a loop, so your solution doesn't work with them. It just occured to me is a way to stop the shipping rules from being accumulative.