2 Aspects to this Lee that people seem to be picking up on. The first - and most relevant - is whether the discount you are offering is reasonable on the car you are selling. 10% is a fairly decent offer but I will be honest and say that I'm the type of person that always looks for just a bit more. I think if you offered some courtesies such as a free full tank of fuel or some paintwork protection then it might be the icing on the cake. I do think however that in terms of actual *money off* I think you're deal is pretty good.
The second aspect is the nigh-on-incredulous prices of the cars. I have to be honest and say that £27k for a diesel Golf is a bit daft. In no way do I wish to criticise the GTD as it is a fantastic car but at the end of the day, it's a 2.0 TDI Golf with different seats, a bodykit and some badges. I have heard recently the advert on the radio for the "fantastic VW value" which lets people get into a new Golf for £14k. I have to laugh when I then read about a diesel Golf priced just under £30k. So with the spec list you provided and a budget of £30k I went shopping...
Here's what I found (all spec'd brand new on the manufacturer's website). Where I was unable to specify an option I have mentioned it:
BMW 320d ES Saloon (182HP) minus colour multifunction display, flat tyre indicator and dynaudio £29,980
BMW 120d ES Coupe (174HP) minus colour multifunction display, flat tyre indicator and dynaudio £26,175
BMW 120d Sport Hatch (181HP) £27,770
Audi A3 Sport 2.0 TDI (170HP) £25,585
Audi A3 Sport 2.0 TDI (170HP) Quattro minus colour multifunction display £27,130
Seat Leon FR 2.0 TDI (170HP) minus colour multifunction display, flat tyre indicator and dynaudio £21,710
Vauxhall Astra GTC SRI 2.0 CDTI (165PS) minus colour multifunction display, cruise control and flat tyre indicator £23,560
Ford Focus Titanium X 2.0 TDCI (163PS) minus colour multifunction display and flat tyre indicator but with metallic paint £22,745
*These are just list prices and quite possibly could be haggled to lower figures.
Now I started the list with some cars which are more in keeping with the 'prestige' of the GTD badge. BMW don't do proper 'M' diesels (not yet anyway) so I just picked an engine with the closest amount of power to the Golf. I then progressed to the A3 and the FR - both siblings to the Golf - although Audi don't do a special badged diesel model. I then looked at cars which might compete with the Golf in terms of size although if I'm honest then the Vauxhall and Ford can probably be dismissed on terms of quality.
My point however is that for the nigh on £30k that VW want for that particularly spec'd GTD, there are some seriously tempting options from other marques and the cars which are perhaps the truest rivals to the Golf (namely the Astra and the Focus) are both seriously cheaper.
Wake up VW!