It's perhaps worth beginning to pull together why the consensus here is that the OP isn't in a strong position.
If I have understood things correctly, you booked an 'Advance' ticket from Redland to Ruabon. I am assuming that by 'Advance' ticket you don't just mean one that you bought a while before travelling, but one as described
here (the National Rail website description of an Advance ticket). As that page says, the great thing about those tickets is that
Advance fares are Single (one-way) tickets offering great value for money on many longer distance journeys.
but the big drawback of them is that
Advance fares are valid only on the date and train shown on the ticket and seat reservation and are non-refundable.
As you have explained, your journey began to go wrong at Temple Meads. You could see that a connection that you had expected to make at Newport was impossible. So does this mean that the railways would have expected you to get another ticket? No: the railways do understand that connections sometimes don't work, so you would have been entitled to take the next train by the same operator on the same route as the train you were booked to take. (Someone will be along in a minute to point out that this is a very brief summary of the actual rules and there is more to it than this, but I think it's good enough for the moment.)
What actually happened at Temple Meads? You consulted a member of staff on the platform:
3 Platform staff advice to my question “ Can I get to Ruabon via Birmingham by boarding that train as the train to Newport is delayed “ was “ Yes”
Assuming that this is a precise account of what you asked and the answer you received, then the answer was correct, but unhelpful, in that what you actually needed to know was 'could you get to Ruabon via Birmingham
using the ticket that you held?' The answer to that question would have been 'no' because, as I've said above, Advance tickets are only valid on the 'date and train shown on the ticket'.
This is where you fell foul of the rules. Once you left Temple Meads towards Birmingham, you didn't have a valid ticket for the journey that you were making. So the on-train staff were within their rights to require you to get the right sort of ticket to cover your journey.
By the letter of the rules, you are not in a strong position. You were making a journey without having paid for that journey. The railways understandably disapprove of fare evasion and so there are laws that allow the railways to take action that will strongly discourage people from travelling for free more than once. If you decline the railways' current offer to settle for the cost of a ticket plus their investigation costs they have the option of taking you to court. Courts, of course, are staffed by people who might have sympathy for your plight - but these people are also required to uphold the law and may decide that the law takes priority over your view that you have not been fairly treated.
The quick way to bring things to an end is to pay the amount you have been asked for, and write it off to experience. You may be able to get a better result by taking matters forward, but be aware that the next stage is court, and that if you lose there you may lose more heavily than by paying up now.
