Have had the motor out.
The tacho coil on this model is build into the actual motor and looks like it's only possible to remove it by taking the motor apart! Have traced the wires back to the connector and I get a steady resistance across these, no matter how much I wiggle the wiring. Also, if I put a voltmeter across the terminals and spin the armature by hand I get a fluctuating voltage. Obviously, I don't know if the voltage is correct, but it proves that the coil isn't open circuit and that the magnet hasn't fallen off!
I have done a continuity check on the wiring between the tacho and the control PCB and this appears OK.
I have removed the PCB and inspected it and can't see anything wrong, no burnt components, no dry joints. I have cleaned the terminals on the PCB and the motor, for good measure.
Still the problem persists. No error codes or flashing lights.
Obviously, it'd be nice to confirm the fault by substituting parts but a new control board is ~£75 and a secondhand motor is about £50, neither of which I can really justify on such a cheap machine!
(I have video'd the machine misbehaving and might upload it later)
I suspect I'll have to end up breaking the machine and selling the bits to recoup my outlay; shame as it's in A1 condition and hasn't seem much use.
Edit: It doesn't exhibit the issue during 'tumble dry' mode, nor does it do it on 'spin', although it doesn't spin if heavily loaded.
Control module= Minisel 546052300-002
Motor= 512011603 (The newer square version, not the old round type)