Craigy.... The sprocket retaining nut can be reached with the chain removed, a homespun crowsfoot two prong tool (like a 2 pin angle grinder wrench) should be enough to see if the nut is loose, as CJ reckons. It might be finger tight, but in any event, with side movement, tightening up the nut is the fix. No need for extra spacers. This would save a lot of hassle removing clutch and primary drive, and is awkward to do especially if the tab washer is in the way. At least you will have found the root cause, how it is fixed is up to you, short term hack or major surgery. Either way, it's a down in the dirt job.
Swarfy.