Hi Roy,
I just had a quick look on autotrader, there are two tt aero tops for sale. One in silver 69k miles almost totally standard for £12,750. It seems that the red one has been reduced from £11.5k to £10.5k (its been on there for quite some time) it got 59k miles and is also totally standard apart from some cheap looking alloys. There was another one up at 14k as Scooter mentioned, it must have been sold v. quickly - don't think its been on there longer than a week.
So given that, I reckon you could go up to 15k, perhaps 14k. I haven't seen a black aerotop tt on there for ages and believe me, I've been looking! If you put your car up at a higher price (14-15k), the chances are buyers would percieve your car to be of superior quality. Hope you get a quick sale.
Cheers,
Joe