My Mum has tickets - bought through the RailUK site - to travel from Penally to Huddersfield on Thurs 15th Dec. The itinerary on her ticket is to leave PNA at 07:24 and arrive at HUD at 15:08.
It is looking like the 07:24 train is cancelled so the next one is at 09:34 arriving at 17:08.
Obviously it would be better if the 07:24 runs: is that still a possibility?
No, as the network doesn't reopen until around 07:00 on the days following strike days. That's the time trains can start to leave their depots/stabling locations - so Penally doesn't get its first eastbound train until 09:34, once the first train of the day from Carmarthen (retimed to 07:14) has reached Pembroke Dock and turned back.
If not, is there any reason around revised timetables and strike action why delay repay wouldn't be applicable, given that she has an itinerary proving the existence of the train at the point of booking?
It would also be possible to prove the existence of the train at the time of booking using alternative means, but in short - yes and no.
The National Rail Conditions of travel (NRCoT) were changed earlier this year to include a new definition called the "Published Timetable of the Day" (PToD). This definition essentially purports that the timetable advertised at National Rail Enquiries' website (nre.co.uk) at 22:00 the day before travel is the one that counts for the purposes of delay compensation. In other words, the timetable in place at the time of booking is irrelevant for the purpose of delay compensation.
There's been plenty of discussion on this subject - searching "published timetable of the day site:railforums.co.uk" ought to turn up plenty of threads. Suffice it to say, my view is that this is an unfair and hence unenforceable term, given the context of a consumer contract.
However, that doesn't stop operators from dealing with your claim in line with what the NRCoT say. And thus it quite likely that to obtain the recourse your mother deserves, she would have to take legal action. Whether that would be worth it for the amount of compensation in question would be down to her to decide.
There is also an entitlement to delay compensation, albeit a reduce level compared to what is usually payable under Delay Repay schemes, under the Passenger Rights and Obligations Regulation (PRO - again you can read previous threads on this). The PRO specifically states that entitlements under the Regulation cannot be reduced or eliminated by conditions of carriage. However, again this is an example where most operators seem to think they are above the law, and hence you would likely need to take them to court to get what you are owed.
It's a very unsatisfactory position, but unfortunately the rail industry has been doing this with impunity since the PToD concept was introduced - and until there's political intervention, I can't see that changing.