I learn't a bit more about it yesterday. It's blown the two parallel heater fuses before and indeed that'd happened again. New fuses held until I went to 4th fan speed and then popped promptly. (lower speeds didn't work anyway)
Rooted about a bit with the heater fan wiring, the 2 pin plugs at the fan motor itself and the overlaid A/C wiring have been a bit toasty but that's not highly unusual for a car of this age.
Without a load lamp to diagnose with I resorted to overrating the fuses. Factory 2 x 15a is a 30a total... so I put a 25 + 30 and then went looking. Short bursts of turning the fan switch on allowed me to check voltages at the fan & the fan speed resistor. The telling clue was the burning smell coming from the vents even with a only a second or two of fan.
Pulled the heater fan resistor out, lo and behold it was full of leaves and bugs and shit. More interestingly, it has a thermal cutout built into it, most novel. Seems when things get a bit hectic, the solder melts off this component and it then springs free of the resistor circuit... and short circuits to the frame... ffs!
Stuck it back together with a bit of solder, reassemble, replace correct fuses and away she went.
However I still don't fully understand how the circuit works. Seems to have an accessory supply to the fan and then speeds are switched to ground?
Anyhoo... we'll carry on for now. Think I'll rip the heater fan and resistor out of the wreck before it goes to the wrecker.