I have no personal experience of PASS PLUS but a few quotes from their website may prove useful to you...
"If you passed your test more than a year ago you should check that you�ll be eligible for the discounts on offer from your insurance company before you take the course."
"When you have successfully completed the course, you are advised to check available discounts offered by the insurance companies taking part in the scheme. The amount you save will depend on the company you choose."
http://www.passplus.org.uk/about_pp.asp