Math PhD viva (UK): what to expect My viva is coming up in a few days and I'm a bit worried. I'm in a UK institution. 

I want to know how technical will the questions be? I won't be able to remember complicated proofs even though I wrote them, and even though I am going through my thesis. It's a painful process re-reading everything.

Surprisingly, there is no information online about math PhD vivas in the UK. So I have no idea what to expect.
You will not find such information as there is no standard process in the UK. The best people to talk to are your supervisor (who should have knowledge of both examiners, and can tell you what they're like) and the internal examiner (who should be making sure the exam follows your institutions' regulations, and guiding the process).

In UK vivas that I've been involved with, the process is to keep going until the candidate has proved they


did the work that the thesis in their name describes;
can say why it's important;
can say why it's original;
can convince the examiners it's correct.


Typically the external examiner will take the lead in asking questions; they can go into as much depth as they like. The more confident you can make them that you know what you're doing, the less likely they are to drill into tiny details (unless they are particularly interested, in which case it's more a chat than an exam).

Finally, it's now become standard in the cases I've been involved with that the candidate has a few minutes at the beginning to summarize what they did - typically 2-5 minutes. If you get this chance, make sure you take it and can immediately (and I mean, in the first or second sentence out of your mouth), cover the "important" and "original" points above. A confident (in appearance) statement of "This is the important, interesting stuff that I did, and we're now going to have an exciting 2 hour chat about it", gets everybody started on the right footing, builds confidence in all, and usually leads to a good result.