Hang on.
This is confusing.
Goods vehicles under 7.5T are 50, 60, 70 on single, dual and motorway respectively. That's interesting, as I thought ( wrongly, apparently ) that motorways and dual carriageways were the same.
Seems I
can travel at 70 on a motorway.
According to what I can find, the magic figure is
two tonnes Max GVW. , ie fully loaded. Seems all Transits ( including Tourneos, which is another area of confusion ) are in the same class.
There is another interesting pitfall also. When your little van is first registered, it may be listed simply as a van, as opposed to car derived van. Thus, two identical vehicles can have different speed limits!
At least having read the rules again, it seems I can do 70 on the motorways after all.
Please don't take this as the definitive answer, but a government site is good enough for me.