Sorry if I've missed this in the thread and perhaps you're already doing it but . .

a) preferably refuse to speak to them on the phone and insist their complaints are issued in writing; much easier to demonstrate the reasonableness of their response

b) if you must do it by phone make sure you have the name of the woman you're talking to and her line manager (I presume she's told you the company she works for) and if you feel she's being unreasonable (unlikely, I agree) insist on speaking to the LM.

I think the line is "I don't think I am doing anything illegal or unlawful and nothing you have said has changed that opinion. I believe I have taken all reasonable steps to operate within the law. You need to be more specific."

Good luck.

G.