After 3 years lab work (including 2 changes of supervisors - change of institution as one boss left and took me with him but I remained registered with the original uni) and 6 months of write up. I am absolutely terrified of my viva!

I just keep coming up with horror scenarious in my head which isnt helping. The proposed date is the 10th June (my external could only do 3 days in the next 3 months!!). Apart from keeping uptodate with the literature and making sure I know my thesis inside out - what can I do? Im really worried im going to fail - even though I know my supervisors would not let me submit unless i was going to pass

I am not at that stage yet, Littleowls, but what I am planning to do is to gather lists of typical viva questions (you can find them online) and ask friends/family/supervisor to ask me questions, so I get used to articulating my answers. Your supervisor might be able to do a mock viva with you. If you are comfortable giving talks, some people swear by giving a departmental talk shortly before the viva, again to sharpen your focus. I would also say try to do some stuff that's relaxing and remember you're the expert on your thesis!

First of all, congrats on submiting your thesis!!!!

Secondly, I'll tell you how I prepared for my viva..... I read my thesis 3 times and I knew it would be inside out. Then I made a list with all the viva questions that I could find. I also wrote answers to each question. The next step was to find all the answers in my thesis and fill my thesis with notes and post-its (and when I say I filled it I mean I FILLED it! There were so many little papers tham my examiners smiled when I took my copy out of my bag!). I also prepared a list of corrections, eventhough my supervisor advised me not to (for no particular reason).
It really helpd that I had a mock viva with my supervisor and another academic, eventhough it was just 2hours before the viva.
That was all! That was my preparation!
During the viva I had my thesis in front of me (with my post-its) and my notes with the answers yo the potential questions. The list of corrections was in my bag.
The viva lasted about an hour and I passed with (very) minor corrections....

Good luck!!!!!

The best advice I can give you from my experience is to anticipate the kind of questions your examiners are likely to ask based on their research expertise. Also, be wary of your internal. Often students assume that it is the external who calls the shots but I found my internal also put me through her paces! Whatever the research interests are of your examiners, expect questions emerging from this. Good luck!