Sobering story Basil (gulp).
You mentioned emergency bags. Also as a keen winter backpacker and winter fell walker (sorry for using the W word), I usually carry some sort of emergency shelter at the bottom of my pack - lately a Rab survival-zone bivvy. But that's going to be too heavy for my bumbag when running. I have an unopened, foil sealed, plastic survival bag, bought well back into the last millenium, which weighs in at approx. 100 gms. Carried for decades but never required. Is that the sort of kit you would typically carry while running out on the fells during the winter?
By the way, the instructions on the rear of the packaging inform me that it'll also make a handy desert still, so that should keep the 'spirits' up if my navigation really goes wayward.