Pleased to say that I found the problem, the cause of the problem, and fixed it without much hassle.
The problem was water ingress on the wiring plug to the sensor, causing a bit of blue corrosion to both sides of the connection...
...and the cause of the less that perfect seal must go back about five years or so when I got a local garage to change those rear bushes on the front lower arms.
Presumably they disconnected the sensor and somehow managed to kink the seal on the plug. The wiring was also incorrectly routed and not attaached to the bracket where it should be...
My theory is that the corrosion was causing a less than perfect connection and the force of the splashing water moves the wiring slightly (due to it not being located properly on the supporting bracket) causing a very brief break and corresponding fault code logged.
I've fitted a new height sensor, cleaned the corrosion from the wiring side of the connection with contact cleaner and attached that wiring to the bracket as it should be. All connection seals had a smear of my favourite smelly waterproof grease, as well as the corrector linkage.
I cleaned any muck from behind the wheel arch liner, lathered inner wing with Waxoyl, put back together, quick BSI reset when reconnecting the battery, and all is well.
It's not a bad job to do really. Access is tight and getting the small retaining bolt back in took many attempts and several uses of every available swear word.
It'd have been a five minute job if I'd had ET's fingers
)