I've just got compensation out of the Abbey for an endowment taken out in 1985. It's not a lot but neither is my mortgage. I used a company called Keypoint endowment claims
http://www.keypointclaims.com/ As previously said there is quite a lot of form filling and you have to evidence how the endowment was mis-sold. Then there is the fee - 20%.
The company took all the hassle out of the claim but on reflection I could have done the job myself and been 20% better off.
This is a very useful link and does not cost you anything.
http://www.fscs.org.uk/consumer/FAQs/Endowment_claims/ Make the claim, if they say no then at least you tried.
My next job is to buy premium bonds with the compensation money - who knows!