Do not assume because your bill is £35 a month, that them charging you £5 will be a 10% increase. It does not work like that.

They go off your last bill.

If you rang 5 0870 numbers, and together they cost you £6, add that to your bill that is £41. Then lets say the next month you DO NOT ring any 0870 numbers so you bill is £35, but you get charged £5 for data, your bill would be £40. Compare that to the previous month and it DOES NOT increase your bill by 10% so you won't be able to cancel.

Be aware of that!