wrote:

After the usual extensive fuss in which I was threatened with
prosecution for long term fare evasion, I left the scene and got my
train as usual, but I was just wondering, does the inspector have any
grounds for this claim at all? Theoretically, would I have to get off
the train at Putney and then get back on again for the ticket to be
valid?

No. As seems to be happening far too often, the inspector is talking
nonsense, and being unacceptably heavy-handed about doing so as well.

http://www.southernrailway.com/main.php?page_id=218 contains a relevant
extract from the NCoC, or specifically :-
"You may use two or more tickets to travel on one train journey as long
as together they cover the entire journey and one of the following
applies:-

(a) they are zonal tickets;"

That appears to cover it nicely.

Neil