Did you ever get a response from FGW▸ about this, Graham? Are they following it up with the taxi company? I'm still fuming about it!
A response from a senior manager with whom I was discussing wider matters yesterday - verbal and I'm paraphrasing - "what happened [as reported] was absolutely wrong - and we will make sure that there's a reminder / instruction given to bus and taxi drivers. Please ensure that we hear about it with specifics if it happens again.". When pressed, "there may be a legal argument that if they don't have a ticket already there's no contract ... but there is NO WAY that First would look to deny carriage"; the manager went on to identify things like dreadful press that could result ...
In summary, I'm going to suggest an over-zealous or under-instructed contractor driving the rail replacement service, or elements of the situation that you / I are not aware of as they were absent from reports.
But - yes - unless the driver felt that he was threatened, I'm still very much of the opinion that what happened was dreadful ... "if that had been my daughter left behind ...."