Fixed after THREE days in the main dealers.
The problem was that it was intermittent and unpredictable, so they had to do repeat testing to be sure it was cleared.
After they bled the brakes I collected it and drove 250 yards before it happened again, straight back in and booked it in for next day (Tuesday). Got it back Thursday.
Two front sensors were caked with dirt and salt and badly corroded, hence the feel of ABS cutting in when it shouldn't. They replaced them and (said) that they charged me staff rates for the sensors and labour (discount for "good customer"). Total bill £168.
Probably didn't cover the cost of their time really, but I'm afraid I was rather irate at the time it took and complained at being charged at all for a fault which I (at first) thought they had caused with the brake fluid change.