Just though I'd let you know what's happened so far:
1) Went £25 over my o/d limit
2) Charged £25 for paying a direct debit
3) Charged 25 unauthorised o/d fee
4) Charged 2nd unauthorised o/d fee on day 5 despite not being over my limit as of day 5!
5) I'm now REALLY not happy as I would have let them get away with just the 1st £25...
6) Send initial letter saying it's not fair charging £75 for going £25 over my limit etc.
7) Get standard reply saying these charges as in their terms and conditions and they will not refund

I'm now past not happy and a bit p'd off, search on internet and find this brilliant site via
http://www.moneysavingexpert.com - another great site!
9) As I've already sent a polite and friendly letter and got nowhere I sent my version of the standard letter on this site giving them 14 days to refund in full or I'll see you in court - felt a bit uneasy about doing it but as soon as it was sent I felt MUCH better, at least I could kick them in the shins if I couldn't get any higher!
10) Within a week I recieved a proper letter (not a standard one I think) saying they are investigating my complaint, they enclosed a leaflet which basically was stalling for time, so I will stick to the 14 days I gave them in my not so nice letter - it's great to be in control....
11) Waiting for 14 days to be up and i'll fill in my online court form so I might be asking for some advice then