My point is that I would never ever pay the screen price.

Here is another little tip to find out how much, give or take a few quid, they have paid for the vehicle. Have a look on there website for any car that is about £10k more than the C63. Ring them to enquire about said car and tell them you have a C63 PP with, say 15k miles on, to part exchange. Tell them you live far away and would prefer a rough valuation over the phone so that you knew what to expect if you went through to have a look at it.
When you then go through to purchase your C63 you can barter in the knowledge that you know how much they roughly paid for it.

I would say they have about £30-34k in the car. Good luck anyway and I hope you can save a few quid in the process...