As you say Stuart, very difficult to gauge a price. If it was something like a B6 S4 Avant in Imola Yellow then it would be different and treated as a classic. Reality is it’s a B6 3.0 litre petrol, 23 years old with 109k on the clock with crusty wings.
Going to break your heart here, but it’s worth more to you than it’s physically worth. On positive, belts done, year’s MOT, tow bar, and leather. That’s cheap motoring for someone. Start at £1,200 and see if you get any interest. Try Gumtree. You could swap out wings, but I’m not sure that makes it more saleable.
There’s a private B7 (2008) 3.0TDI Avant on Autotrader, badged as S4 but it isn’t, lowered, 160k miles, service history, paint immaculate for £2k.