The meeting lasted about 10 minutes. My union rep advised me that taking into account a tax and NI deduction (which I was eligible for) I should have been able to knock it down to about £4000. The HR rep came straight out with an apology about how it is the payrolls mistake but they are still going to reclaim some money. She came out with a value that was half of what's on the letter. I consulted my union rep and he was satisfied it was the best offer we could probably get without taking it to court (which I didn't want, by law the courts would have favoured them and I would also have to pay court fees).
So a not so exciting ending. I'm paying back half of what I was overpaid.