Unless you have it insured 'new for old' it may be that the insurer reduces the sum insured each year and hence the lower premium.
It is always worth checking the market value of your van each year just before renewal, as this is all that you would get in the event of a total loss claim. If you don't do this you may be paying a higher premium than necessary.
It is sometimes difficult to establish market value for older and/or foreign vans - my own is a case in point. One way is to trawl the 'For Sale' sites for a similar van and copy any particulars that are found - the 'Save as...' facility on most browsers makes this easy. Then you could print these off in the event of having to negotiaste a claim.