I have 2 accounts with the RBS and they can't transfer money from one to the other withour me asking them toi, and so i doubt they could snatch any of your pre-paid funds.
Get the new o2 one if you have an o2 mobile, no charges for transactions at all.
I think RBS's terms and conditions probably do allow them to take your money from one account to offset another, Dot, but they'd do this only in exceptional circumstances.
It is almost always part of the small print that banks can offset a debt on one account against a credit on another. This doesn't just apply to current accounts. It could be a credit card debt offset against a current account balance. Whether RBS can do this with a NatWest account I don't know, but it wouldn't surprise me if they can.