PPI with Halifax BS

I submitted an enquiry to Halifax about claiming PPI on the joint mortgage my mother and I had for the period form 1988 to 2000. However, they sent me a ridiculously long form asking for details of account numbers, copy mortgage statements, details of the mortgage product we had, etc etc. I just looked at the form and thought it was pointless trying to fill it in because it was over 18 years ago and I haven't got records going back that far. What makes it even more complicated is that the original mortgage was taken out with Leeds Permanent Building Society in 1988, which Halifax BS took over sometime in the 1990s. I would really like to make a claim against the PPI we paid on our mortgage because we were told we had to take it out and also we were charged interest on the monthly premiums so for 12 years we probably paid a lot more interest than we should have done.

Is there anyway we can make a claim without being able to provide the information they reckon they need? Would I be better using a claim handler to sort this out given the age of it? If so, are there any recommendations of any decent claim handlers and what fee would I expect to pay. Also, for submitting a claim, does my mother need to sign papers? She now has dementia and is living in a care home. I have Enduring Power of Attorney for managing her affairs so assume that will be sufficient.