It's a difficult one - I sort of agree with all the last 3 comments, not very helpful I know!
If your heart is set on that particular car, I guess I'd begrudgingly pay a little extra, but not much, as goram says really, but no more.
It's the principle of already having agreed the deal with the company, they now want to change it. And that's wrong
But I would do absolutely nothing for the time being - let them make the first move. They will at some point.
Provided you're in no rush just sit it out - play them at their own game. It's a tried and tested sales tactic to get you to make the first move. Don't do it!
I'll be absolutely stunned if they're prepared to lose a sale for the sake of £300, and it's a buyer's market so you're in the 'driving' seat (sorry!).
If you get fed up of waiting ask for your money back - that'll then be their last chance to sell you the car.
As swq4 says, there may well be a better one out there for you anyway