Would you really cancel the order for a new car because your baby seat wont fit? Would it not be best to get the car, take it to mothercare, then go from there? Could you not sell the seat 2nd hand and buy one that does fit properly?
I have had a read up on it though and apparently the child seat manufacturers only test the seats in a certain number of cars, then only approve the seat for that car. According to the guidelines you should be able to contact maxi-cosi for a list of models they approve. It will still fit in other isofix cars however.