I've done some research for you and cannot find Susan, David or Michelle on any electoral roll lists between 1998-2004. Maybe they emigrated somewhere?
However, I have found a possible address for Martin, their son.
If you would care to contact me direct, I will give you the information I have found. If it turns out to be correct, he would presumably know where his parents and sister were living.
Regards, Avril firstname.lastname@example.org