Catrin
is there a possibility of photocopying the following section of music and sticking it alongside the first so you don't need to turn at that percise moment?
This breaks copyright rules, but given the circumstances you may be ok if (1) you explain to the examiner why you've done so, (2) you destroy the extra piece of paper after you play it - in front of the examiner.
I think if you wrote to the publishers explaining this problem, they'd give you permission, and if they gave you permission in writing, it would look even more impressive on the exam day - how about contacting the publishers as a first step?
However understanding they may be, examiners will listen to the entire piece as a performance at this level in my view, and if it stops at a crucial moment for a page turn, it will upset the performance as a whole.
Furthermore, it's VERY easy to lose track if you stop at the wrong moment, fiddle, then continue. I've been in this situation performing live, and what happened once - the page turn didn't go well, and I was left fumbling with the book in my left hand while playing 'solo' on the right, for a couple of bars! Very embarassing and very obvious to the listeners! I then thought I'd get my 5yr old to help the next time, and she was so keen to do her only page turn, she dropped the entire book off the piano, leaving me with no music at all