I was extremely happy with the amounts that I got which included 8% interest backdated.
The first I heard back from them were the cheques in the post. This was a few years ago and last year I received another cheque out of the blue when one of the banks revised the amount I was due
Do it yourself if you can. I had PPI with two banks so filled out the template on the Money saving expert site then sent them recorded.
I had a cheque from each bank within 4 weeks.