[tin]

I'd advise against wasting your time on the phone altogether. Write a letter. State clearly your position in the least amount of words reasonably possible in quick bullet points, then state what you expect them to do. If a letter comes back, in reply or unsolicited, reply to that person/department with the same information. If it happens more than a couple of times, state the same info and that you'll invoice £60/hour for your future services in dealing with the matter.

It's much more final in a letter, it's harder to ignore, and if you keep it brief it's easy for them to see where you stand and what you expect.

I did this and eventually got left alone (no refund was due, fortunately), but it was MUCH less hassle than worrying about it and spending ages on the phone either on hold or speaking to people who did not understand/couldn't care less.