Of course, the Rail Delivery Group could remove all doubt by publically re-iterating the convention that if a train is not shown in the public timetable as having first class accommodation, any first class accommodation is effectively declassified. This would simplify things for everyone, staff and passengers included.
Come on RDG - isn't simplification like this what you're always calling for ?
It would be clarification, not simplification.
It's pretty obvious that a Guard cannot overrule advertised terms & conditions or consumer laws. National Rail Enquiries says "This train has Standard Class seating only" and that is definitive. It cannot be over-ruled by someone acting far beyond their authority.
Nevertheless, I did once encounter a GWR Guard who claimed that a Penzance to Plymouth train contained 1st class accommodation even though the timetable stated otherwise. I called Customer Services, who agreed he was wrong to be charging people Weekend First upgrades. I put him on my phone to them, they told him the facts, and he disagreed and started arguing with them! Unbelievable behaviour.
I now wish I had paid for an upgrade myself as I would not only have been able to obtain a refund but I could have ensured the matter was appropriately dealt with and a suitable letter of apology was received. I'll be sure to do that in future if the situation arises. There are still a
small minority of Guards who are of the view 'it's my train, my rules apply' and believe they can disregard contract law, consumer law, and the relevant T&Cs, but this is completely inappropriate behaviour and it needs to be reported and then dealt with.