After 18 months (including a bad experience which made me change supervisors after 1yr) I am finding my heart simply isn't in it anymore. I feel that these 18 months were a mistake, and I've decided that I will probably quit after my MPhil. But does quitting a PhD with an MPhil imply that I simply wasn’t good enough to complete? How do I communicate that this is not why I left? How do I ask my supervisor to be a referee without causing problems during my MPhil?

I have heard a MPhil is seen as a failed PhD, but given they hit you with the Overqualified label after you are finished with a PhD, who cares. Prepare for hundreds of job applications in either case and a lot of arrogant managers wanting to smash your achievments.

An MPhil is good in my book :-)

Unfortunately MPhils are awarded both to those who decide not to continue and complete a PhD and to those who do complete, but aren't considered good enough for the award. This can devalue the achievements of those who have simply decided not to continue.

Fortunately, outside academia or jobs which require a PhD, very few people know what an MPhil is and will think you are extremely intelligent either way. If you wish to start a second PhD, then you'll be able to explain to your potential new boss why you submitted early the first time around. Whilst they may be put off and think you might pack it in again, they won't think you weren't good enough first time around.

Some students set out to do an MPhil as opposed to a PhD because they don't have the time and/or money for a 3 year course.