So ... an update.
I've finally gotten around to working on this problem.
I raised the back end off the ground, started her up and ran it in gear so the rear tyre is spinning.
I got no signal out of the ECU on the speed signal wire so I thought I'd see if I could get anything directly off the road speed sensor. All I got out of it was 5V DC. No square wave at all on the pink wire.
TuneECU shows 0kmh no matter how fast the rear wheel is spinning. In retrospect, I should've checked that first.
So after all that, I'm thinking the road speed sensor is stuffed.
I presume this is the little bugger hiding behind the sidestand:
On the bright side, I worked out what the signal is on the tacho signal wire.
12V DC when you turn the ignition on then 0 - 12V square wave once engine fires
80ms between pulses (12.5Hz) at 750RPM so 1 cycle per revolution of the crank
I'm not sure if the 12VDC is supplied by the tacho and the ECU just pulls it down to 0V or if the ECU is sending the full signal itself. I didn't want to cut that wire until I've confirmed I'm going to replace the gauges.
If the speed sensor is the only problem then I'll retain the stock clocks. If I can get my hands on another loom for the gauges, I'll cut it and find out where the DC is coming from.