This is a common problem. The easiest way to keep track of the topups is to register a debit/credit card with the network and use that for topups. At least the actual payments will then be seen on the statements.
Failing that, if you need to use cash, our local garage gives a little slip to people who ask for a receipt for a topup done with a swipe card so maybe you can find somewhere nearby.
If all else fails just keep a manual track of the cash. As long as it is reasonable you should have no issues. I have not seen HMRC look too closely at phone expenses unless they are abnormally high - but then you would have changed back to contract for cheapness!