Update, with pictures!
I did the obvious, and checked behind the dash panel. Somebody had clearly yanked it out at some stage and detached pretty much everything, possibly looking to solve the fault described below.
So on plugging the hazard switch back in, some really odd behaviour emerges. I *do* get hazards and indication (albeit with 3 apparently dead bulbs). However, the flasher relay clicks all the time. Now I see why someone unplugged everything! So when you push the indicator stalk left, you get left indication. Push right, you get right indication. Restore it to the rest position, and you get no indication, but the relay is still clicking.
Does this likely suggest the relay is dodgy, a leaky earth, or something more sinister? I've attached a pic of the offending object it's a Bosch P122 Electronic Flasher - never seen one like this before, but I guess they come in different shapes and sizes...?
Captain clicky relay: two pins...
It looks compatible with the right Volvo one?
Somebody got mad. Real mad. Much unplug.