Drumroll.............
sorry I didnt get back earlier, been to busy wheeling again
It turned out to be a dodgy idler pulley. To the naked eye it didnt wobble but over a few seconds when under tension it must have been starting to twist off the bearing center ever so slightly, it was flipped around and would be ok for a bit but would then slightly lean outwards again throwing it off. Undetectable to the naked eye but throwing a new one on has solved the problem..Thanks for all your replies fellers.
Cheers..