Thats crap mate - I'd ask the chap since he should have declared their true value with is somewhere around the price of an empty bottle of coke
You do not have to pay if you are re-exporting I didn't think. Call HMRC and tell them the situation.
I got screwed when Tamiya USA sent me some parts FREE. I lost count but I must have pair around £180 in vat/duty/charges for something that was sent to me. In that case I called hmrc to dispute it since they were samples for review - but because I wasn't going to re-export it I think that was the problem.
You are - so I don't think the VAT should apply. It's like you're fixing something for someone, you're improving it, and sending it back. I'm fairly sure you can talk to hmrc and get a refund give these circumstances.
My recent situation was different since I bought something and should have paid tax etc. But it was so grossly miscalculated that I refused. They wanted so much proof of the costs of shipping that I've no proof of (since it was included) so they just make up a number and multiplied by 10.