Hi Gareth, I tend to agree with that approach, to let it find its price on eBay, but if I was to advertise I think I'd start at £3,900.
If nothing else, I see that 4.2 TDI replacement engines are in the £4k - £5k range and given that my car has such a good engine and gearbox, plus a load of gadgets, I would expect that offering it at £3,900 would be reasonable. That way someone could invest a bit of time & money to bring it back to being a £10k car, or in the worst case could recoup the money in parts.