Deore trigger shifters on the Dawes. They were failing to ratchet reliably in the minus-cold temperatures of Tan Hill at the weekend, but managed a partial recovery after 3 hours on a Pendolino in time for the ride across Brum. In light of this, I followed Torslanda's advice and removed the cable threading hole plug, inserted the straw from my can of GT85 and flushed them through with copious quanties of the stuff, interspersed with repeated frobbing. They're now clicking firmly and were completely reliable on a short ride to the shops at a tropical 7C (final judgement suspended pending colder conditions).
Also swapped the Marathon Winters back to Duremes and gave the rear wheel a quick session in the truing stand (I suspect one of the spokes had had a kerfuffle with a D-lock at some point, as it was just the one that needed significant tweaking).