There were some 1 tonne Sankeys built to carry 1,000 litre bowser tanks, but IIRC they were non-tailgate narrow types. Whether any Widetracks were buiolt as 1 tonne I don't know. They do look big though and being the full width of the body you can get lots of stuff in, probably more than they can carry.
You should have a built or data plate somewhere though :-k that siad my 1tonne Rapier was re-rated by the REME to 1,300kg payload as it wasn't used off-road like it was designed.