Well despite the underwhelmng response, I finally got round to sorting the loose platform. Have flu so can't work hard, or even think hard.
The engine plate bolts appear to have been tightened by someone more used to tractors.... After jury rigging extensions to various spanners/rings the magdyno came off intact, using a 40 year unused puller.
Contrary to all written advice I didn't remove the timing chest cover or even loosen it. I merely had to slip off the Dyno on refitting; after cleaning and loctiting the four screws, I used an impact for good measure. I initially scribed a mark for the points so I didn't have to retime the mag.
The biggest time waster was getting the nut back behind the clutch housing. I finally pushed a ring up with the captive nut and after tightening managed to slip the ring off, just....
Anyway it turned out my bike had a vertical slot in the back of the timing chest casting just behind the mag sprocket,thus negating the need to remove said casting.
Is this normal?

Yes that slot is present on all timing chests. Don't know where you read to remove the timing chest but it's not necessary, it is of course required to remove the timing chest cover.
Not so sure about the scribed line to keep the timing. As soon as you pull the mag chain sprocket off the timing is lost.