In simple,
You shouldn't have to pay more if it was a genuine mistake.
If you pass your station, or board the wrong train. Immidiately seek out the on-train ticket inspector and explain your problem, ask their advice and produce the tickets you have. Give a sincere apology and they should direct you to a train home and mark the back of your ticket to let the next inspector know (most TI's will do this, if you're genuine).
Once you get on the second train, seek out the TI again, explain what's happened, produce your tickets.
You should be okay. Worse comes to worse, you get a £20 PF or have to pay a return fare, just turns out to be a costly mistake.
Yours,
Sparky-