Had the Focus into my Ford Dealer and came back with no fault found. The changed all the wheel sensors, but the problem persists.
Then, I was chatting to a guy at work who has his own garage. I described the symptoms, and he was convinced it was the battery that was on its way out.
He suggested that I turn on the engine, and all high load items such as lights, heated screens etc. If the voltage at the battery was above 13.5v he was sure it would be the battery.
It was 14.5v.
New battery fitted, and all cured.
Seemingly when the PCM detects low battery voltage it can throw up all sorts of weird fault codes.
So if you guys ever get the same weird behaviour, you'll know where yo look.