This morning I sorted out the belt tensioner. I cut the end off the tensioner where the bearing had fitted. Turned a sleeve to fit the I.D. of the insert that goes in the pulley bearing. Then put it all together.
I refitted the tensioner to tractor, same way as before and gave it a try. Now the flat of the belt was running in the V of the pulley. Not really an Ideal thing, plus the belt flapped a bit. So a bit of thinking and trying different positions, here is what I ended up with.
Seems to work fine and as soon as I can I'll give it a trial.