So the true story is that snowfall brings dangerous roads & even more dangerous drivers. The city councils seem to think it's a good idea to plough the roads (fair enough) & then spread sand/grit & salt on them to increase grip. The result is, the cars pick up the slushy mix of snow & grit with their tyres & deposit it under the wheel arches. Typically these deposits fall of when they get too big & fall off. It seems to be that shopping centre car parks are the most popular place. I guess the heat of the engine bay melts the snow & it simply slips off.
Thanks to my mate Jim M. for alerting me to the phenomenon & the terminology.
