Yorkie - while I'm not overly comfortable about SWT charging a PF in this case, you could argue that they were entitled to.
From the SWT site :-
If you are travelling from a penalty fare station on a penalty fare train, and:
a)
You do not have a valid ticket and are approached by a revenue protection employee, who is an authorised penalty fare collector you may be liable to a penalty fare of £20 or twice the full single fare - whichever is the greater amount to the next station at which your train stops.
Now, we know of the exceptions regarding being off route or travelling outside of validity in terms of time, but neither exception applies here. They were having a BoJ in effect which wasn't permitted by the T&Cs they'd agreed to, thereby invalidating the ticket.
To me, the excess fare idea is a non-starter as it surely can't be possible to excess from a Megatrain ticket to the SOS.
Was it morally right for them to be chinged 114 quid? Probably not. Were the SWT RPIs acting within their remit? I rather fear they were