Thats good advice, thankyou.I don't think it's worth answering such emails punch for punch. Whatever you have charged on the invoice for the bath is the cost of that material. Show yourself reasonable, do it right as best as you possibly can, consult a solicitor if need be as to what reasonable would be, and if they actually bother to take you to court, so be it. They can only win a court case if you have not done your legal duties (your responsibilities, as you correctly put it), not just win because they are still dissatisfied.
As for expecting cash, that is reasonable if that is how they originally paid you, but you need a receipt then. Quite what you would do with their bank details (name, sort code, and account number) apart from pay money in is beyond me. (Try going to Tesco and paying by card and then expecting a refund in cash and see how far that gets you). You don't need to justify all this. You can merely say that you will offer the refund by the same method it was paid to you as that is your policy. If that isn't good enough for them but it's an offer that is within the rules of the law (which I expect it is), then how are you being unreasonable?
You can't please some people all of the time.
Ive offered 25% over the price of the bath, and l charge £220 a day labour only, so offered either my time free of chatge to put their new bath in, or another £250 to chose anither Plumber to fit it, l said this would only be paid via bank transfer as that is how they paid, plus it is proof they have had it.
My thoughts are they will go down the small claims route, but surely, if l can prove l offered a resolution they would not approve a much higher claim in monetary value??