Come on breaker breaker, don't get excited, it's only advice?
Check that the motor is working, if it is, then just replace the gearing, if not the whole thing, after checking that the shock has not seized.
I'm 99% sure it was the motor that had failed - I swapped it for another which worked fine so the electrics seem ok - I've also now opened up the actuator and can confirm that the plastic planetary gears are all fine. The cog on all the shock absorbers rotate freely through about 180 degrees.
It was the nearside rear (RHD) which is a bit unusual.
With a bit of luck I may at last get rid of that pesky ERROR light