Hang on a minute, on the one hand they are saying that the divorce info was input in 1993 and the person who did it is dead, and now theyre saying that they sent you that letter before the divorce info was input.
I would write a very strong letter t them, sent via recorded delivery and also send one to your solicitors so they can see cc- solicitors company on the letter, saying that it is virtually impossible to prove a negative, but that you are happy to make an affadavit to the effect that you were legally married etc and that they have 14 day to respond apprpriately or you will turn it over to your solicitor and their costs will naturally be added to any claim you make against them.