Delving a bit deeper today; got the centre hub off but held the chainwheel in place to observe the bearings and noticed a bigger than normal gap i.e. One roller missing!
And no, I didn't mislay one! Getting it on the bench, everything measures correctly, no wear or damage anywhere so I therefore conclude that the lateral play I was concerned about, is built in and "normal". It's a Triumph clutch and unbelievably, the extractor I bought back in the late '50's for my T100, fitted, which was a bit of a result.
Also exposed the oil seal in the crankcase, which looks ok. I now have to decide whether or not I go the whole hog and have the gearbox out to cure the oil leak.
Decisions, decisions🤔