Argh, spoke too soon w.r.t. my first correction! Missed off the end of
a sentence here...

Mizter T wrote:
[snip]
(2) FCC's regular Off-Peak Day Return fare is £17 - this limits you to
travel down and back on FCC Thameslink trains, so your departure point
will be one of the central London Thameslink stations (St Pancras down
to London Bridge). This isn't a special offer, and as a regular fare
all Railcard discounts are available as normal. You can buy this

....from the ticket office on the day, or perhaps even from a ticket
machine if you can get it to co-operate and offer what you want.

(Fairly obvious point that is implied above - this is for travel on
FCC trains only.)