Most insurers will ask you the value of the car, so if you've splashed out on a £36K 330d with all the toys you might put down £48K. However, IMHO that's purely so they can load your premium because when it comes to paying out they look at the book value of the base spec car, then subtract a random amount to make the lowest offer they think they can get away with. When my father-in-law's new fully loaded Merc was stolen, they offered him £12K under the base book value. When challenged they said "that's all you're getting". When they eventually got a solicitor's letter they suddenly apologized and said they'd mixed their figures up. Thieving...
PS - I've worked for an insurance company in the past... never again. I was actually insured with them through a staff scheme and when a no-fault accident landed me in hospital and wrecked my motor they had no qualms in screwing me over!!