Hi Paul,
I think you have made the right choice.
I have used Carjet several times without any problems. The one thing you do need to do is make sure the car can be collected at the airport.
Carjet are only brokers and some of the companies they use ie Auriga are off airport and can put up to1 hour on your transfer time.
If you are not sure on this point then just email them to make sure.
Regards
David & Gill