Unfortunately there's no such thing as its true worth.
For £2.95 you can get a detailed valuation from glasses guide but this is only a guide. The extra's on it don't add any significant value to the car, just make it easier to sell.
If you find someone looking for that particular model/colour/trim then you'll get slightly more for it (i.e. they won't knock you down on price that much), but it all depends on who's looking for that car in your area at the time.
You've got the trade in price which is about £9k, which is the rock bottom crap price, but you could get as much as £12k for it privately.
Looking on autotrader there's not that many around, but all the £13-£15k ones are around 50k miles, and of course that doesn't mean they'll sell at that price.
I sold my wife's '99 A3TS for £6.1k last month, which was priced according to the glass's valuation @ £6.9k, but there were 200 other similar models available varying from £3-£10k according to mileage. I only had 2 calls on it in 3 weeks, (apart from all the canvassers). Luckily the buyer knew exactly what he wanted, an unmolested genuine low mileage example that had been well loved but not fiddled with, so I got what I wanted for it, although he did knock me down £800 (as cambelt was coming up, and front dics/pads were getting low, there was no CD changer).
I would say with your spec is perfect (it's not green, it's got leather, nice wheels) so you'll have the bulk of the S4 Avant market interested. Whatever you do don't get a price from a dealer as it'll knock your confidence, I was offered £4.5k which to be fair was exactly right according to the glass's valuation.
