My mum and dad went through quite a hard time when I was younger with finance. Mother slipped a disc in her back and lost her job. My dad was self-employed and had 2 heart attacks etc. Long story short, they had to sell their house and then paid off their loans with the money they had left. They had a dreadful time with the phone constantly ringing with people chasing debt.
My dad wouldn't do it (put in a PPI claim) as it caused him too much stress. I convinced my mum to do this as a 'punt'. She was not happy about doing it, but agreed as I said what's the worst that can happen. She was convinced she had never had it. Anyway, turns out she had, and got some back. Then I managed to convince my dad to try. Again, sure he never had it. He had. Got over £8k back so far (with a few still to hear from). Both, genuinely never know they had it. Basically completed forms saying as much.
Cannot describe how happy I am for them because they have always struggled with money. When I heard from my dad he said he was "literally still shaking" after finding out. I cannot recommend checking with banks enough to see if you had it. At the worst, you never had it. I'd say I spent about 2 hours total, checking with providers and filling out claim forms. Barclays easiest to deal with again.