I took GIII piano last Wednesday and was a total bag of nerves.
I had only got about 6 bars into my first piece and completely lost it !
Luckily, the examiner was very sympathetic and put me at ease.
He told me to calm down, relax and take a minute. Then he said 'now start again'
At the end, I asked him if it really sounded as bad as I thought and he said no.
He told me that it was obvious that I enjoyed playing piano, it's just nerves and it wasn't as bad as I thought
Haven't got my results yet so I don't know what the outcome will be.
I think if you can make an overall good impression and the examiner can read between the nerves, I don't think it probably matters that you start again.
Suppose it depends on the examiner too.
Fiona