UPDATE:
Had the amp connected up tonight, still with initial strange noises when ignition isnt turned past 0. As I turned the key from position 0 to 1 it made the speakers output a scratching noise (and a "pop"), like interference as I turned it - this makes me think... bad earth/dirty ignition switch contacts. Once at position 1 it was fine just as before. I drove the car to a friend's house which took about 20 mins - he is quite technically minded and I was trying to demonstrate the problem to him, however when I turned the ignition off/removed key, the problem didnt occur?? Which immediately made me think that perhaps the problem is looking more likely to do with the battery, as my train of thought would be that I had driven it for 20+ mins, therefore putting a little more life into the battery, and perhaps a low voltage was causing the amp to play up beforehand? So I locked the car, went inside with the amp still connected since it seemed to be fine, and when I went back out, unlocked and it was fine for about 20-30 seconds, then the noise came back again. Drove home, and once again took key out and again, the noise didnt come back. I have disconnected it tonight anyway so that I don't wake up to a dead car!! Can any electrical wizards confirm whether this looks more likely to be a battery related problem or a bad earth... or perhaps something totally different? I will do the battery test tomorrow and see how it holds up under a bit of electrical strain. Any advice would be fantastic.