I thought National Rail terms and conditions on Saver tickets
allowed a break of journey on the return journey.

How come LUL don't appear to agree that where the saver is to
Underground Zones 1 and 2, issued by One at Cambridge? WAGN staff I
checked with confirmed my recollection of the rules.

From the 'Conditions of Carriage' (1.C.15).

"Please note that a ticket which entitles you to travel on the London
Underground does not entitle you to break and resume your journey at
any of its stations unless it is a season ticket or a Travelcard."

Oh sh*t!

:-)

I only became aware of it because I was using a tube between Farringdon and
Liverpool Street and decided to hop off at Moorgate to pop into M&S. As
Moorgate is extremely close to Liverpool Street I had no intention of
resuming my journey on the tube. My gate pass would not operate the exit
gates at Moorgate. Fair enough I suppose though I can't see what there
would be to lose by it allowing me to exit but not to enter again.