Good luck with the sale Matt, however im sure you dont enjoy being 'humoured', that is far too expensive IMO, if your not in a rush great, if you want to sell it - you need to take other cars that have recently sold into account...firstly the yellow interior is a marmite thing even more than it would be normally as it isnt a yellow car - secondly, it isnt a unique colour, probably one of the most common colour S3's you can buy...
re the comment above £1500 extra over trade for the extras? thats sales my friend....your car is not worth £1500 over trade for anything, he was feeding you a line and dipping into his profit if he told you this and proved that the trade price was accurate (glasses or CAP guide), either that or he undercut the trade price to start with which is normally the case, its called managing expectations, easier to add value than take it away
Sorry to be harsh but that is the way it is im afraid.... i wish you no hardship in selling your car....
Tim