Hi Andrew.

Just been through the mill with this on my new toy. If you don't have any luck, it might be worth trying the fix I'm using.

Before dismantling anything and assuming the existing tensioner spring device is apparently exerting a broadly appropriate tension, measure the distance between the spring device bolt centres.

Remove the spring device and also the pivot assembly. withdraw the pivot bearing centre sleeve and check for the wear ridges; rotate through 90 deg to present a fresh face to the needles and refit.
I then cut a piece of 8mm steel to broadly replicate the overall area of the spring device. Using the bolt centre measurements, I drilled an appropriate size (8mm bit) hole at one end and then milled a very short slot at the other. Fitted new belt in place (piece of cake with no tension on). Bolt but don't tighten the plate to the pivot assembly.
Next bit isn't for the faint hearted. Using a short jemmy, lever down on top of the bearing housing in the tensioning pulley (not on the belt face) against the chassis rail. Have your bottom bolt ready and fit when the slot lines up with the hole. (Takes a fair bit of pressure on the jemmy to achieve this) Check the belt tension. I settled for being able to twist it about a third of a turn between the crank and the air con pulley. Using the jemmy to maintain pressure if required, tighten the bolts.

On mine I think I have got reasonable tension on the belt and the big bonus is the steel plate has trued up the pulley wheel. As it is no longer rocking to and fro, I'm hoping it is a reasonably long term fix. In the fullness of time, I'm hoping to knock up a mechanical adjustment rod to replace the plate. I'll just have to wait and see if the absence of the tensioning device has any adverse effects. Meanwhile though, absolutely no signs of adverse wear patterns on the new belt at all.
For reference, I used QH belt QBR61863 which seems A!. Didn't replace the actual tensioning pulley wheel as the bearing in it was spot on.

Mike.