IMO getting your car detailed when new is a bit of a waste of your wonga yes they use top products and all know there stuff but id save the 250 for in a years time. On the new car theres going to be minimal paint contamination it will be in need of a clean and plenty of wax and sealant which you can buy / do yourself. Then in a year when your cars paint could do with a refresh spend your money as it will be more worthwhile due to having a years worth of crap built up. I cant help but feel for £250 you paying for something that you dont need.
On the other hand if you dont want to put the time in yourself or have plenty to throw at it then fair enough.