Should you decide to write to them (based on what you say and have/photo evidence , there is no case, no claim so no need to write, just ignore), then add a confirmation that you are charging them £25 to cover your time and expense of notifying them to drop their illicit claim , and that you have no intention of paying, and should they continue to write/threaten you, then you will charge them an additional £25 henceforward for each and every one of your written replies.This should do the trick. I never return telephone calls from such people , as they are usually a waste of your time/money . One letter detailing your charges is more effective , they will no doubt decide to pick on some other unfortunate instead.
I have used this technique with various people and agencies over the years, and also last month with my previous broadband provider who thought they could charge me £75 broadband retrospective equipment charges - they have now dropped that claim, have also given me a refund of advance rental charges, and I am awaiting £75 costs from them for the time/3 letters I wrote making them aware of their illicit and erroneous claims for payment.
My wife says that as a pensioner I have too much time on my hands, that may be so but a person who works for free is always in work, and should you not place a value on your time, then generally nobody else will