its not because the car has gone up, the on the road price has now increased because it now costs £38 instead of £25 to register a car and they have added that cost on to the on the road price of the car. They probably can charge you the extra as its not their fault the government did it, but to keep a customer happy I would be very surprised if they made you pay the extra £13, not worth the hassle, will cost them more in admin charges!