Ahh!! Got it. Sorry for that, I realise how it all works together now and what the 12mm does. I have put in the new alternator but I had to squeeze and shove the belt a fair bit. It has loads of tiny hairline cracks in but I mean really really small. The belt doesn't have to be sitting in the same place as before right? Just in the grooves.
Started car and the red battery light is off thankfully, and got 14.45v at alternator and 14.3v at battery. Excellent!! Got the alt from GMH vehicle recycling in Cardiff, nice guy and very fast devilery. All for £40. So I would recommend him.
Thanks for all your help people!