They have to respond within 5 working days.
They then have 8 weeks to deal with your complaint.
What they will do is look at what happened when you were sold the policy - were alternatives discussed (ie were you aware that you could have repayment mortgage instead of endowment?). What was your attitude to investment risk, and was the fund that your money was invested in suitable for your assessed attitude to risk? Did they make you aware that the policy might not pay off the mortgage - or did they state or imply that it would not only pay it off in full, but would also give you a lump sum.
They should also look at other more "administrative" issues - does the policy run into your (or your partner's) retirement, does the sum assured match the mortgage amount, was the policy written on the correct lives, did you have any other policies that you were encouraged to cancel that could have been used to support a mortgage?
They will look at evidence from the time of the sale (fact find document, application form, illustration), and also should ask you what your recollection is of the sale, as well as the adviser who sold the policy.
If they decide that you were mis-sold the policy, they will redress you so that you are in the position you would have been in if you had not bought the endowment. It's a bit complicated (ie I don't really understand it all myself), but the most common way is they look at how much of a repayment mortgage you would have paid off by now, against the surrender value of the policy, and give you the difference.
Hope that makes some sort of sense - if you've got any other questions, I'll do my best to answer.