Well firstly by saying that someone could perhaps then say the driver is not being truthful.
Why would anyone say the driver wasn't being truthful? He is alleged to have said something which undermines his own position.
Secondly that in law bit doesn't make sense remember innocent till proven guilty , so if someone said I did something wrong you believe them but by the same token then presumes I'm guilty so calling me a liar .
No, people are believed by default when they make a complaint. That's how everything works. It is the glue of our society. If I make a complaint to the bus company that I was discriminated against, then by default I am believed, and the incident investigated, unless there is a compelling reason to suppose I am lying. (for example: Perhaps I'm a serial liar known to them, or perhaps I'm unknown to them but there was no bus at all and I made it all up and it is obvious from the outset, or perhaps my lies come out during the investigation.)
If I tell the police that I was mugged, then again by default I am believed, and the incident investigated, unless there is a compelling reason to suppose I am lying.
People are presumed to be telling the truth at all times Baz - even people on the witness stand in a court, defendant and plaintiff - unless it can be shown that they are being untruthful. Key - it has to be *shown*.
If you are accused of a murder you didn't commit, and you end up in court for it, and plead not guilty the law presumes you are telling the truth - innocent until proven otherwise. That does not mean that the prosecution are de facto liars. Everything the prosecution and the defence say is presumed to be truthful in court, unless it can be shown (sigh) that it is not truthful. It is then up to the jury to decide on whether the person is guilty - note that this is different to deciding on who is and isn't a liar.
Civic society doesn't work if people don't believe other people by default. This is the starting position for any interaction in society, unless there is a good reason to suspect the party is untruthful (this might be because of past history and context - e.g. take certain stories from the Daily Mail with a heavy pinch of salt).
Hence, we should believe the baroness, because we have no reason to suppose she is being untruthful.
I am astonished, really, that I have to explain the basic tenets of living in a civilised society.
just sticking up for the driver not actually trying to dis credit the baroness.
No, not trying to discredit the baroness at all (!), except for the bit where you said she was wealthy, privileged, and would have tried to get the driver sacked if he caused a delay, and it was "convenient" she saw what she saw.
You can stick up for the driver all you want without doing any of the above (and I note you have started actually attacking the meat of the subject, thankfully), but I'm afraid the sum total of the interaction between driver and baroness was that the us company, and by extension the driver failed in their duty under the law, and the baroness did nothing wrong.