It sounds rather like they are trying to pull a fast one on you. If they have a signed contract between themselves and you to supply the car for £40285 then that's what you should pay.
If they are asking you to sign a new contract then surely they are trying to negotiate a new deal. This being the case I would be siting the sources by which you can get a better deal in todays market and ask them if they really want to start over again or stick to the contract that you both signed. They might want to pass on the cost increases, but you'd want a better discount.
Haggle hard on the px and the best of luck
Matt