You only need to touch in or out of the system when you enter or leave
it. Transferring between lines does not need a touch in/out. If you
are continuing past the limits of the system, then you need to touch
out on joining Thameslink.

Hmm, I think I was having a bit of a brain fart there. I was forgetting
that if National Rail has Pre-Pay it is part of the zonal fares system and
so indeed the equivalent of just changing tube line.

G.