Mystery solved! I was about to have my tyre changed anyway so I had my mechanic dismantle the whole rear wheel assembly and we discovered the rear suspension had been locked at near full compression. There IS a very slight lean to the left, but this is an inherent design flaw in the engine placement of a 200kg bike.
It seems that combined with the slightly worn tyre, zero give on suspension and an intermittent problem with the throttle cable, the back wheel had started to spin with nothing to grab onto until the back end swung out and found purchase on the unworn rubber.
It’s a great ride now, and my arse no longer feels raw after a 10km journey. I love this machine but I’m not entirely sure why… it’s old, it’s dirty, I paid too much for it and it feels like a brick outhouse on wheels. Some idiot rode straight through red lights into my side a few weeks back and bounced off cartoon-style. Then again, it’s super comfortable, doesn’t give two hoots if I stick an extra 12 stone guy on the back and manages a bazillion miles per gallon.