Difficult question. However, regardless of what the list price is now, your Cali won't have gone up in value since you bought it, so what ever it's insured value was then, it should still be more than appropriate now. Anyway, should anything happen to your van and it is written off, your insurance company will only offer you what they feel is the current market value for your vehicle taking into account its age and mileage etc. I am not entirely sure what mechanism they use to work out these values but their first offer is always going to be low. If you are talking about taking out GAP insurance then I am not sure where you would stand with an eighteen month old van. Could you even get GAP insurance on your vehicle at this stage? I don't know.
If you really want to know its current value then all you can do is to look at what other people are asking for their Calis with a similar mileage, spec, year and then decide what you could realistically ask for yours if you were going to sell it. Pay attention to how quickly these other vehicles appear to sell at their advertised prices. Over priced vehicles will hang about for a long time.
To help you decide on that value, also check out the VW Commercial website and Autotrader. Look at what the dealers are asking for similar vehicles. Whilst anyone buying would normally expect to pay less from a private seller, the California market is full of surprises. Don't even bother asking a dealer about trade in values and don't look in Parkers or any other car pricing guide as they are a work of fiction where the Cali is concerned. There is currently a lot of Californias being advertised so that might have some effect on acheivable second hand prices. Remember that regardless of how many options you ticked when ordering, many of them will not improve the second hand value much over a more basic model at point of sale.
Unfortunately, like anything else, in the end it is only worth what someone is prepared to pay for it. If you were thinking of selling but not desparate to do a deal quickly then you could chance your arm and go for a high price. If you van is just what a particular buyer is after you may be lucky and achieve that higher than average price. However, the down side to this strategy is that you may have to wait many weeks or months to find that buyer and in the mean time the market may have changed and the value of your vehicle will also have dropped.
Having recently gone through this exercise, I may be wrong but I would guess that your vans realistic resale value for a private sale would at best be somewhere in the low to mid £50Ks.
I hope that helps.