Mike G
Well I did take the PSU out, and removed the fan. I confirmed that (a) the fan was spinning just fine at high speed when supplied from a 12 volt battery; and (b) there appears to be a thermistor component soldered to the PSU circuit board RIGHT NEXT TO the fan!!
This would appear to be a serious design fault causing premature PSU failure due to incorrect thermistor placement, so the fan is "tricked" into believing the PSU is cool when really it may be boiling down the back end.
Suggested solutions:
(i) Reverse-mount the fan so it "sucks" air in from the back. The side louvres will probably get preference, so the rear components may not get adequate air flow. Furthermore I'd be blowing hot air under the floppy, and towards the HDD;
(ii) Cut the fan's red and black wires close to the PSU circuit board, route them outside the PSU case (easy!) when re-attaching the fan with its 4 bolts, and hot-wire it to a 12 volt line (yellow+. black-) to any of the hard/floppy devices, taking care to cover the soldered connections with long-life insulation tape.
I chose option (ii) and although my machine is now making more whining fan noise, I am confident the PSU will no longer be routinely running over-temperature.
Sheesh HP! Shame on your design flaw....