You'd think it would be a simple answer, but it seems it's not.
I've got a 2004 Doblo Cargo 1.3Mjet, the most basic one you can get with no bulkhead, side doors or anything else to add weight. On the V5 registration document it's a "car derived van" which means it has a GVW of less than 2000kg (I think it's 1975kg, so only just under). That's not what it weighs, but what it could weigh if loaded to the maximum Fiat say it can take. It therefore can do 70mph on a motorway the same as a car can.
There are a number of Doblo versions though that have a GVW over 2000kg. They (I believe) show as something else on the V5 (panel van?) and they're only allowed to do 60mph on the motorway. I'm not sure if any of the 1.3 versions exceed 2000kg, but your V5 will tell you which category the authorities have put it in.
Hope this helps, but if you get a ticket don't blame me - I could be wrong, I have been before!