In reply to SI A:
Defo 100% cotton...same brand as most of the others I wear, and about halfway up the "vintage" list. It's maybe a year old.
I'll try the high temp. We normally do quite low temp washes.
From that suggestion do I take it that it's possible for the bacteria from some "overpoweringly bad" past sweat event to still be lurking in the armpits, even when washed normally?