Thanks Pleiades. I'll start with the cush drive rubbers and check the bearings while I'm at it. My biggest worry was that it was possibly something in the clutch or gearbox causing the chain to slap. I suppose it makes sense to start repairing the easy parts (cush drive rubbers, wheel bearings etc.) and work my way towards the harder things if the noise persists.
Thanks again for your decisive reply. It's given me good direction and a starting point.
I'm sure I'll be back on one wheel in no time