Don't know what a SAR is!

Hi Everyone, Never used this forum before but been a member for sometime just wondering if anyone can help? I am getting very frustrated and disillusioned with claiming PPI. Early last year I took up a several PPI cases against a couple of banks namely HSBC for credit card and mortgage, RBS, Barclay Card and MBNA. I have no records but I do know I definitely had PPI on all of these even though I had Perm Health Insurance and I was employed by the NHS and would have been paid for a year. It goes without saying that all replied negatively asking me for more details that I don’t have as these accounts go back to mid/late 1990s and after moving house a couple of times the documents have been lost/destroyed. Recently I’ve made a claim through a well advertised legal company who promised me they have other ways of extracting information and getting a positive response but appear to have given up at the first hurdle after another negative response requesting further details. I am now retired, a disabled wheelchair user and could do with the windfall who I feel entitled to due to greedy mis-selling. I’ve also asked the banks directly but they won’t help. Can anyone out there help me please? Thanks, Paul