Wrighty just keep your cool in the exam and try not to worry too much (easier said then done). I was in a class of 10 and only 4 of us passed! Alot of these guys worried them self so much and flapped that they cost themselves a lot of time with just worrying. Answer a question then move on and stop worrying about it. You will have time to go over them after im sure, but get SOMETHING down first.
As others have said read GN3 till your eyes bleed, then keep reading at every opportunity. The biggest problem with this exam is that it does not follow a pattern from year to year. Every year the questions differ alot. No amount of 'past papers' is going to show you what questions will be in your exam, just learn the main formulas, the rest of the information will be given to you in the form of tables.
On my B question i had a scenario of a garden lamp post wired into the house and all questions in part B were on that, cpcs, cable calculations, right methods of erection. Just a tip in part A you will probably be asked about different parts of a supply from generator at power station to end loading equipment. Do a full diagram and make sure you know the correct terminology because they mark you down for using wrong terminology so you can really gain some marks on a clear CORRECTLY labelled diagram. And CORRECT terminolgy in the whole exam can really get you marks.
Last bit of advice, if you dont have it yet, the 2391 city and guilds exam book is a must. tells you what to expect in the exam and the layout of it, then couple of practice questions. i advise it.
Good luck with it mate, im sure your pass, just stop worrying
