Thanks
@Stevo7682, I've got it off today to replace with a part from eurocarparts. I've only got one tensioner pulley on the M44, mine doesn't have the idler pulley for some reason.
Unfortunately the part supplied was wrong and I couldn't see the right part on their site so I've ended up taking the dust covers off either side of the original pulley wheel, checked the bearings are all there and in good nick, then filled the ball race with new grease and reinstalled it. The noise has now gone and its sounding sweet now! Hopefully it'll stay quiet for a while before needing replacement.