PPI - Payment Protection Insurance
Note that the time limit, mentioned frequently in those annoying PPI ads is 29th August 2019.
Making a PPI claim is easy - either do it yourself or get help from a charity which does not charge.
Both Which and MSE have helpful support on how to check and claim PPI refunds. CAB has a full explanation of PPI (including when it might actually be a good thing), it then goes on to recommend using MSE (a CAB sponsor).
What is PPI
and what is misselling?
From the 1980s, the banks heavily promoted insurance on credit agreements (i.e. credit cards, loans and mortgages) as an easy way of making money as it often adds 20% to the overall repayment. The "insurance" was supposed to offer protection if the insured suffered reduced earnings through illness or loss of their job, but in reality, around a third of policies were unfit for purpose. After a series of consumer campaigns (largely by Which and CAB) and legal actions, the banks eventually caved in and started the mammoth task of refunding payments plus interest thereon.
The PPI scandal has been disastrous for the banks both financially and in terms of reputation. Having built a discredited PPI marketing industry, the banks have been forced to build much larger and more expensive PPI refund departments. The refunds, interest and processing costs have run in to the billions: an article in Wikipedia estimates a total of £26.5 billion by January 2017.
The campaign has been such a success that it has given rise to another unedifying industry, offering unnecessary and expensive services "helping" individuals to claim their refunds and trousering a large share of the proceeds. (Not to mention the annoyance of their TV and radio advertisements.)
Again, making a PPI claim is easy - either do it yourself using the Which? or MSE links above or get help from a charity which does not charge.