OK, solved it now. Ignoring the fact that I'm using a Duet V0.6 and reading the documentation for the Duet 2 family:
https://docs.duet3d.com/Duet3D_hardware/Duet_2_family/Duet_2_WiFi_Ethernet_Hardware_Overview#inputoutput
There's a table on that page listing logical Logical pins (RRF 2.x) against Pin names (RRF 3.x) and that gives fan[0-2] mappings to the logical number range 20-22.

So by adding the following to my configuration file:

M106 P0 I-1   ;disable normal FAN0 operation
M571 S1.0 P20 ;Set 100% output to FAN 0 on extrude

It's all sorted now. I guess the firmware was transitioning to version 2 around this time and the logical pin numbering was already in place but not fully documented? Panic over anyway.

Hi @dc42
Oh, for some reason I had the impression that V2.x firmware was incompatible with the very first (V0.6) Duet hardware.

So, just to be clear, flashing with Version 2.05.1 will not create any issues? Then what about upgrading from there to V3.x?

I'm still occasionally printing things with my Ormerod which also has a Duet V0.6 controller and would like to keep it as up-to-date as possible. It still uses the inductive Z probe that you may recall from our experiments with various technologies in the early days.

OK, solved it now. Ignoring the fact that I'm using a Duet V0.6 and reading the documentation for the Duet 2 family:
https://docs.duet3d.com/Duet3D_hardware/Duet_2_family/Duet_2_WiFi_Ethernet_Hardware_Overview#inputoutput
There's a table on that page listing logical Logical pins (RRF 2.x) against Pin names (RRF 3.x) and that gives fan[0-2] mappings to the logical number range 20-22.

So by adding the following to my configuration file:

M106 P0 I-1   ;disable normal FAN0 operation
M571 S1.0 P20 ;Set 100% output to FAN 0 on extrude

It's all sorted now. I guess the firmware was transitioning to version 2 around this time and the logical pin numbering was already in place but not fully documented? Panic over anyway.

Hi. After many orbits or the Sun I dug out my old laser cutter fitted with an early Duet (V0.6) and flashed the controller with the "ultimate" DC42 firmware compatible with that board I think (1.26.1)

My previous technique using a much earlier firmware was to use "M571 Set output on extrude" to enable the laser from the FAN0 output but that no longer works.
Looking at the Notes for M571 in [docs.duet3d.com] it says:

"RepRapFirmware 1.20 and later do not default to using the FAN0 output, so you must send M571 with a P parameter at least once to define the pin that you wish to use."

Ok, but what logical pin number activates FAN0? All the tables of logical pin numbers I can find only list signals on the expansion header.
I confirmed that Set output on extrude works in principle by issuing M571 S1.0 P65 and that does indeed drive pin 35 on the expansion header high during extrusion but I need an inverted signal, and I liked using FAN0 mosfet switching to ground when active for this purpose.

If it was possible to invert the logic on the available pins like P65 that'd be OK but I can't see a way to do this either.