If "Customer Refund Service" is the name of a company that specialises in PPI claims then avoid them and make a claim yourself. This information, from the Government-funded Money Advice Service, explains how to go about it:
https://www.moneyadviceservice.org.uk/en/articles/reclaiming-payment-protection-insurance
However if the 'Customer Refund Service' is actually part of the bank or building society that provided you with your mortgage, go ahead with their service. (That would seem to be unlikely though if they're not already in possession of all the relevant information).
I got a letter, out of the blue, from what looked like just another PPI firm touting for business. I ignored it for several months, partly because although I'd had PPI I didn't think that it had actually been mis-sold to me. I then took another look at the letter and, after a bit of web-searching, I realised that it had come from the American bank which had taken over the credit card company I'd used. It invited me to go online and enter a reference number to see if I could claim. I did so purely out of curiosity, expecting to be confronted with lots of questions I couldn't answer. Instead I immediately got a message saying "Thank you for submitting your claim, which we will process within the next few weeks". 3 weeks later I received a cheque for around £3000!