I've just been checking what the syllabus says about the Singing at Sight part of the exam, and it says:

Candidates will be given a short interval of up to half a minute in which to look through and, if they wish, try out any part of the test before they are required to perform it for assessment.

I'm expecting a resounding "NO" to this question, but I'll ask it anyway...... does this mean I can try out any tricky bits on the piano, or at least play myself the key chord so I can pitch the notes in my head?

Nope, you can only try it out yourself on the instrument that's being examined. The examiner will give you your starting note.

Oh well, worth a try!