How should I deal with this?

I would call up and explain and then return the communication with a covering letter, the cheapest way you can that you know it will be guaranteed delivery/. Keep a receipt of postage and a copy of the letter you have sent.You shouldn't really have to do this but at the end of the day it could cause you much hle and embarrment if you don't. If after all that the firm still harr you get a solicitors letter sent and request they reimburse the fee.