It doesn't need to be 100% accurate for insurance. If you've only just bought it use the purchase price plus a little bit. If you've had it for a while look on Autotrader and places like that for similar vans and use the higher end of the prices you find. As long as you're reasonably close it won't make any difference to your premium. It's a replacement value figure so whatever you tell them will be meaningless in 3 months time and anyway if the worst does happen they will ignore whatever figure you gave them and make you an offer based on the current market replacement value.