"Mizter T" wrote

Coming from the south a London Terminals ticket is in fact valid as
far as City Thameslink.

At Blackfriars, despite it being gated, there should be Oyster readers
somewhere on the platform - perhaps near the subway - so passengers
transferring from Southeastern trains on a rail-only ticket can touch-
in their Oyster and thus continue travelling northwards on the
Thameslink route without needing to exit and re-enter the gates. You
could thus get off at Blackfriars and try and do this, perhaps even
getting back on the same train.

Assuming the normal Thameslink Sunday timetables apply, even if you can't
make it back on to teh same train at Blackfriars after touching in, the
train from Sutton should be 5 minutes behind the train from Brighton, so it
won't waste much time to 'rebook'.

Peter