Letter structure, btw, I'd just go with somethign on the lines of:
Further to our conversation, I must decline the ridiculous off of £200 to "move on", and fully expect you to:
1) calculate exactly by how much I have been overcharged and refund this.
2) immediately adjust the interest and payments so that I am no longer overcharged.
3) make an adequate offer of compensation, bearing in mind the amount of which I have been deprived which I could have used to gain interest elsewhere.
4) investigate how this could have happened in the first place.
Something like that anyway.

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