Get their address and write to them; explain that they have already been informed in writing that, as a third party, this is nothing to do with you.
Give them 14 days, from the date of your letter, to desist calling.
Tell them that after that date *any* sort of communication from them will attract a £ 25 administration charge and you will be billing them on a weekly basis.
If they *do* continue to contact you, make a note of time and date of telephone calls (and any other pertinent details) and draw up an invoice and send it to them.
But I think you'll be pleasantly surprised to find that they stop ringing.............:o)