does anyone know any statistics of how many PhD students drop out, how many fail their viva, how many have to take a second viva etc?

Don't have any stats or a reference but I was told at a pre-viva workshop that, by far, the most common scenario is a pass with minor corrections.

I was told that 1 in 4 people who start a PhD don't finish it.

But most of those are people who drop out or get part way through and decide to take an MPhil.

But very few get failed.

That was what I was told in a seminar

I was also told that if you start it, and stick to it, you'll (probably) get the PhD. It would be good to have some actual statistics though