Today I.....
Removed the front pulley, rear pulley and drive belt. I then secured the rear pulley in my Black and Decker workmate with the belt in place and, with a piece of softwood as a drift and a hammer began tapping the belt into place to the required 1.5 inches depth, every few seconds tightening or replacing a zip tie and retightening until it could be tightened no more.
I then lightly sanded the clutch drum and shoes with 120 grit paper before cleaning everything and then offering it up to the rear drive as well as looping it around the front drive. I loctited and tightened the rear clutch to the required torque and, with the belt still zip tied, I offered, re-offered and offered again the front drive plate up to the drive shaft whilst simultaneously pinching the belt to gain maximum clearance. Each time I heard a definite 'metal on metal' sound so I loctited the threads and tightened the retaining nut to the torque in the manual.
I then reassembled the covers etc and took the DT out for a 25 mile or so run. (the 'howling' normally started around 12 miles or so). I did several stop and starts before taking it to a steep hill in town where I pulled over halfway up, waited for a gap in the traffic and then gunned the throttle. Normally this would have it howling like a banshee and......
....Success!
The DT flew up the hill without any unnecessary or annoying sounds.
So, I get there in the end. I'm a bit like whitewater rafting. Challenging at first, but if you stick at it, ultimately rewarding.
Many thanks to Stig and particularly Karl for their patience.