I hope this helps; I have dimensions diagrams for both classes.
'N': Loco - 61tons 4cwt Tender - 42tons 8cwt. Total 103t.12c.
'U': Loco - 62tons 6cwt Tender - 42tons 8cwt. Total 104t.14c.
I assume that the additional 1ton 2cwt was due to the 'U' being a 3" bigger engine in height (12'10" as opposed to 12'7"). The wheels would certainly be heavier as 6' on the 'U' and 5'6" on the 'N'. The frames would likely be a bit heavier on the 'U' and, of course, it had splashers as well.
Nice engines and good to see the film of 31806.
Incidentally, the 'U1' was only 1ton 1cwt heavier than the 'N1'!
Best wishes.
John