I've got £250 compulsory and £0 voluntary. As i have other cars insured with different companies, i got them all to quote and for me and the companies that quoted lower compulsory, also quoted higher premiums (by around 200~250). I'm well over 25 with lots of NCB (used on the Golf) by the way. My other cars have £0 compulsory excess so i'm assuming in my case, the insurance group of the car has a big effect.