when she gets home, ask her

did you keep a copy of the letter you sent?
who did you send the letter to?
old keeper?
new keeper?
house in general?
names? addresses? for corespondence etc?
in any case, how old is your son? if he's old enough they'll chase him for it, but seeing as you have tried to get the window repaired and have gone to the trouble of having the glazier out etc and the window was broken BEFORE they moved in and there was enough time for it to be sorted at YOUR expense

in any case, i'd write them a polite letter in return asking wtf they are talking about, at least you haven't ignored them but at the same time you are not implicating yourself in the matter either