Commute home last night had a visit from the fairy at the rear. Felt round inside the tyre, couldn't find anything poking through, so replaced the tube, carried on. About 5 miles later and a second rapid loss of the rear, close enough that it was quicker to carry on walking rather than ride home.
Checked the tyre, 2cm gash in the sidewall, no spare tire at home, so ordered a replacement, in the meantime booted the tire to get me to and from the station this morning - but the hole is showing as a 2mm gap through which the boot is visible, so I'm not going to rely on it lasting particularly long.