Whatever pay level you are on now add 2% to your yearly wage (thats what you get on promotion 2% pay rise), then look at the pay scales for high band Cpl and whichever is the closest new pay increment (without going below your current pay + 2% total) is your new pay scale increment.
current pay is £23580.00 add 2% = £24051.60
pay scales (Not actual figures, plucked from thin air):
L4 = £23850.00
L5 = £24320.00
You will then move to Level 5 as this is the closest without going below your old pay + 2% which would now be £24051.60