Okay so... a little update.
When looking at the idler pulley, it was slightly on the ****, and the bottom looked like it was touching the cambelt housing. I took the pulley off and there was a groove on the cambelt housing where the wheel must of been rubbing/ rattling.
I decided, as the wheel seemed in good condition, to space it out by 1-2mm with a small washer. And for the most part my rattle is now gone, I could here it every so often but it's no longer a constant irritating rattle. I think I would require a bigger washer to eradicate it completely but for the time being it's an improvement.
Now, onto my next issue. Quietening down the above rattle, I can now hear a rumble/ vibration coming from the aux. On inspection the tensioner vibrating, going up and down at a fast rate and it cannot be stopped when force is applied. The noise is by no means much of an irritation but is this normal?? I would imagine due to it being spring loaded that the fundamental idea is for it to be able to flex or absorb vibrations in the belt..
Does anyone else's tensioner vibrate and cause an audible noise like mine?