I want to use my current money in my account for my new monthly roll on

[Seb5]

30,000

[MI5 · Accepted solution]

@Seb5 

Rolling plans cannot use credit.

If you want to use credit, switch to a big bundle.

Pay As You Go: Introducing "Rolling Plans" 

https://www.o2.co.uk/help/pay-as-you-go/big-bundles

[pgn]

@Seb5 - you can switch from Rolling Plan to Big Bundle on demand using your MyO2 - 

https://accounts.o2.co.uk/signin 

Then you can switch back to Rolling Plan once your credit has been depleted, more info here:

Changing tariffs & adding Bolt Ons

You can switch between Rolling plans and Big Bundle tariffs once a month by visiting My O2.

If you decide to switch from a Rolling plan to one of our Big Bundle tariffs, we’ll cancel your next Rolling plan payment. You’ll be able to carry on using the Rolling plan allowances you have until that was due. Then, when the plan you have expires, we’ll switch you to the Big Bundle you chose.   When that happens, you’ll need to have enough top-up credit on your phone to buy your bundle. Until you do, any calls, texts or data you use will be charged at the standard rates. 

You can choose to switch to a different Rolling plan any time. When you do, we’ll take the payment for your new plan amount straight away – and when your current plan ends, we’ll start you on the new one automatically. After that, your Rolling plan payment will renew as normal each month. 

https://www.o2.co.uk/help/pay-as-you-go/rolling-plans

Good luck.

[MI5]

As included in the link already posted.

[pgn]

Albeit in condensed format:

The Rolling Plans page explains it better.

[MI5]

Which is also included on the same loink.