Paul Corfield wrote:

Not quite correct I'm afraid. The change does apply from today. The fare
to each station in Zone 1 will be the same from say Haywards Heath.
However you must purchase a ticket to your destination station and
cannot use it to a station further on even if the fare is the same. For
example a ticket issued to Farringdon cannot be used to Kings Cross
Thameslink even though the fare will be the same. If you did travel
beyond Farringdon and encountered a ticket inspection you would be
subject to an excess / penalty fare (I don't know if Penalty Fares
operate on Thameslink or not).

Thanks Paul. And yeah, they do operate a Penalty Fares system, which I
was caught out by on this very issue in the past, hence asking