I completely sympathise with you Craig. I am completely the same.
On my GI piano practical, I was so nervous - shaking even ! But when I walked in the exam room, there was this little old Welsh guy with a huge smile. That made me feel so much better. He also talked to me before hand just generally. So when it came to sitting at the piano, I felt completely at ease and played almost as well as I would at home. Consequently, I got an honours.
My GII was completely different. I had this other guy who was quite straight laced, gave me a quick smile and started the exam immediatley. I was absolutley shaking. I even messed up my best piece which I could do perfectly for months! I just went home and cried.

I did get a merit so at least I passed.
I do think that the examiners could make more of an effort to put you at ease b4 you start. Especially if you are an older student like me.
I agree with Billy. The only thing to really shake the nerves is to invite people around and play a few well played pieces to show off. It's a good confidence booster. I tend to play better and less nervous the more people there are rather than just 1.
You are not alone!
Hope this helps !
Fiona