I was also suspicious so I called Gladstone Brookes and they said they have lots of different departments. I gave them the full number, And they said it was ok.
Gladstone Brookes and they were excellent at getting some of my PPI back, Not all admittedly as accounts were too far back. They gave a great service and followed up with appropriate paperwork and online access for me to track my claims. Everyone I spoke to was very helpful although I did speak to lots of individuals. They change their phone number a lot which is a bit annoying.
However on the whole I would recommend this company.
Gladstone brooks absolutely useless at getting ppi. told me lloydsbank did not know me. went back with my details, they said account number was too old, closed my account in 2010. this was after 5 phone calls from them