Is an MPhil worth anything at all unless you then go for a PhD - I have heard that an MPhil would be regarded as "half a PhD" - is this right?

Outside of academia an MPhil is just another masters degree. Very few people have a clue what they are. In an interview you could try and convince people it's half a PhD and they'd probably believe you, but what would be the point. If you were seeking an academic post then the MPhil would probably not be of much use, but why would you stop at an MPhil if you wanted an academic career?

The problem with an MPhil is that they can be awarded for any number of reasons:

1 - Because, after a year or two of research, the candidate decided that was the degree they should submit a thesis for. Either due to a change in personal circumstances, lack of funding or lack of interest.

2 - Because, after the first year or two of research, the university decided the candidate would not be good enough to complete a PhD and forced them to submit early for an MPhil.

3 - Because, after the first year or two of research, disagreement between the supervisor and the candidate on the progress of the research meant that submitting early for an MPhil was the best option for both parties.

4 - After at least three years research and submitting a thesis for a PhD the candidate, or their thesis, was not considered good enough for the degree of PhD. The examiners can choose to award an MPhil.