Without getting into lots of specifics of particular vehicles, I'm fairly sure for a 2 wheel drive 3.5t vehicle the max you can tow is 2t, and this has to be a 4 wheel braked trailer, this is what my max was with a 3.5t Sprinter when I was a courier towing a two pallet trailer all day.
My 4.6t LT had a GTW of 6.4t, so that would be your 2t, but my Ducato was 2.5t towing for sure.
Initially I thought the Fiat VIN Plate was out of date after conversion because when I checked the Autotrail Brochure, they had a different GTW to the VIN plate, and so I emailed Autotrail for confirmation. They replied back:
"Thank you for your Email
I've checked the towable weights for the 2008 model range which confirms that the maximum towable weight for the Cheyenne 635 was 1250kg.
I hope this clarifies the situation
Kind Regards
Auto Trail Technical Services"
However, I then realised I didn't have a 2008 Model, but the previous years model (it was registered in Early 2008, hence my confusion).
So I emailed them again, this time instead of just asking about the GTW on a general model year, I quoted the unique number of my Autotrail, and they replied back:
"Hello David
Thank you for the follow up e-mail.
We can confirm that the vehicle is of a 2007 season manufacture with a maximum gross vehicle weight of 3500kgs and a maximum Gross Train Weight of 6000kgs as per the Vin plate attached to your vehicle.
Kind regards
PaulB"
So ...3.5t Ducato, 2.5t GTW.
The Autotrail Brochures are actually very informational (that a word?). It provides pretty detailed data for each of their models and each years brochures has information for that year, not generic. Autotrail actually sent me an Excel Spreadsheet with information on the Towable weights for each model, including the differences depending if on the Fiat, Fiat/Alko, MB 315 or MB318 chassis, and also in each case if the quoted Tow Weight can be exceeded or not if the GVW has not been fully utilized.
Given the detail of info provided, I am 100% confident of what they are telling me for my van