Oooh phleb, I don't, but I have done a lot of things in my life, and am happy to share!
I believe you can ask Natwest for past statements, someone told me the banks would charge £10 for a statement so you then have the account number....
The other thing you might do is take a credit reference report (they cost about £3) and all your historic credit should be listed there, with numbers. You still need to be able to prove that they were charging you a PPI premium, though.
A week's a very short space of time - our agency acknowledges in a week but it's usually a month or so after that before anything happens.