In case anyone wonders...
I replaced the washer/wiper switch (no airbag worries) without a result.
I then disconnected the 2 wires at the front washer pump. This seems to have cured the battery drain problem. I've yet to replace the pump, I just swapped the hoses over with the rear pump so I still get front wash for now, albeit by using the rear wash switch.
I'm guessing the front pump must have an internal short & somehow gets key-off power (seems unlikely). The manual is not very clear regarding the washer/wiper wiring system.
I also now keep headlights on for all journeys which forces the charging to stay at 14v.
Most of my journeys are within 3 miles.