Diesel cars do tend to fetch a little more than the equivalent petrol model, however looking at that car it looks mint, provided it's been serviced up and the cam belt has been changed, it's now 40K or 4 years according to Leeds Audi and you can chip say £500 - £600 of it i'd be happy with that.
I paid 11.5K for my 2006 (56), 41K mileage, FSH A3 Sport-back S-LINE TDI 140 back in February this year, it was up at £13000 (too much) and got £1500 knocked off it.
A cars only worth what your prepared to pay, when i bought mine there were cheaper A3s but they had suspicious service history, stone chipped and painted to death or just generally dogs.
I did also purchase an A3 which was described and looked exactly like the one in the link, however when i got it home i soon realised that it was in fact a 140 TDI and not a 170!
I was gutted as it was bright red and just what i'd been looking for with it having Quattro, ended up taking it back and getting my money back in the end and settling the for the 140 i have now, i owned that car for no more than 3 hours, shortest car purchase I've known.