the assessor is there to assess you, not teach you, but as others have said you are perfectly entitled to ask for good clear guidance on what is asked of you, it is not a crime not to understand the assessors request as you are under pressure, any assessor worth his salt will put you at ease and make sure you have every chance of passing, one tip i would offer is on no account ever guess an answer, if you dont know then look it up, i would expect you to be able to do TT, flue flow, spillage etc without thinking, but you wouldnt be the first to ask for MI's to confirm where spillage test must be carried out from, that impresses me more than me not being sure if you had a lucky guess to where the spillage test was done rather than asking, relax and you should be fine, an assessor will suss out in the first 5-10 mins if you know your stuff and will "assist" you as much as you need, if you are full of Bullsh*t you will get hung out to dry