Welcome Glen - I hope you get your questions answered quickly.
Here is a link to a previous post which mentions a company called Freeways
http://www.civinfo.com/forum/12159-post.html
As it says you can buy the car through a broker, like
Buy a Car, Deals on New Cars, drivethedeal.com, New Car Discount, Car Deal Online who can get a fair bit of list price and use Freeways to finance. GlenMH says their rates are good. I don't know if you have a car to PX, thats where it gets a bit messy as you'll have to sell it privately as brokers don't tend to take PX's.
You can also try here
Sainsbury's Bank Home Page under borrowing and car buying.
Anyway hope this helps a bit.