Go to your bank, and talk to your account manager. If they’re any good at all they should do it for you.
I had a word with mine in Lloyds, she gave me a number to call, I got the relevant paperwork sent to me, then dropped it in to her to tick the relevant boxes.
A little while after she phoned me about some issue on one of my accounts, and would I like to pop in and sort it out, then said she’d got my PPI claim back, as well. Sorted out the confusion, then she asked how much I thought I might have got back on my PPI, well, I didn’t have a clue, so she handed me a bit of paper with £11,680 printed on it!
If I’d gone to one of those PPI claim people I’d have probably only got £7000 at most, they keep the rest.