Just to let you know the outcome....
Advantage Stirling reduced the price to £26,500, and paired with the generous trade in they offered, decided to continue with them. Taking into account the trade in, broker fee, travel to the Midlands, etc, actually worked out a few hundered quid cheaper than the brokers.
HOWEVER, I had previously contacted Advantage to advise that I would be cancelling the car. They advised that as the car had been ordered, I would lose my deposit. Not the type to accept this sort of thing, I contacted Consumer Advice (08454 04 05 06,
www.consumerdirect.gov.uk). They confirmed that under the Consumer Credit Act 1974, Section 57, as I had not signed the credit agreement (or even had the finance approved!), the deposit should be returned.
As soon as this was mentioned to Advantage, they were falling over themselves. Turns out they are charged a 2.5% cancellation fee by VW on the cost of the car. Not sure the salesman should have told me that

That also gave me an extra bit of leverage.

Overall, stand your ground, investigate the legalities, contact consumerdirect.gov. They were really helpful.
Thanks for all your help and advice folks.
Just need to wait till September now. The countdown has officially started....
