If it has a CRIS number etched into the window glass then you can work out the age using the following web page: http://www.ukcampsite.co.uk/articles/view.asp?id=89
Otherwise there will be a small plate with numbers on, either on the chassis or maybe under the door area - and I think there is a similar way of identifying the year, using the above system.
Not trying to be critical, but you bought a van without asking its age?
So you obviously knew that the price you paid was relevant to its actual worth?
I assume this was a private purchase because most dealers will give you the age of the van they are selling. A year either way can make a big difference to the value.
As mentioned the works plate will allow you to track its date of construction from the manufacturer.