Late postscript to this saga - as mentioned the M36 belts didn't fit, as they were far too tight and when you engaged the drive you couldn't get the tensioner arm to move, as the belt was so taunt already.
Anyway, 4 mths or so ago, knowing I had the winter break (slight reprieve from the kikuyu monster!) I cut a strip of rubber from a thick section I had - about 10cm long x under 1cm wide - maybe 4mm thick. I carefully threaded this inbetween the belt and the rear drive pulley, effectively trying to stretch the belt by having it pushed out a tad further for several months.
I was pessimistic about how this would go - but surprisingly it achieved enough to allow the drive to be engaged & mow effectively - although I still had to have the tensioner arm in the incorrect spot (applying pressure closer to the middle of the belt rather than the rear. But working is fine by me.
We'll see how long these last - but worked well the first time around.
