I made a complaint regarding PPI on a credit card which I'd taken out over 25 years ago. The bank rejected it so I submitted everything to the financial ombudsman.
This turned into a farce as they mistakenly closed the claim without reviewing it! After a few years of hearing nothing I decided to get in touch with them again, which lead to even more issues as they had to seek some kind of special permissions to reopen the case due to it being closed for so long (some data protection thing I didn't really understand). Anyway, eventually, a year ago and over 5 years since I submitted my original complaint I received a payout of a few quid short of £16,000
Fast forward to now and we are remortgaging to raise funds for an extension to the house. The mortgage advisor hinted that one of the products we have been paying into for around 13 years is no longer available (a form of redundancy protection) and that many have put in PPI claims for that product. So the wife has got the ball rolling and hopefully we'll get a bit of money back for that too.
Definitely worth the effort of doing it yourself.