Digging about in my garage I found a pair of Oxford Muffs. My brother gave them to me years ago but I'd never used then. They consist of a pair of covers for the handlebar grips, made of heavy nylon fabric with a fleecy lining. They keep wind & rain from your hands and provide some insulation from the cold. I've only every seen dispatch riders using them but I thought I'd try them. They fit easily enough needing only a hole made for the left side mirror.
I've been for a couple of runs with them on and they do work. Combined with heated grips they provide a good level of comfort. They look a bit silly but you can't afford to worry about fashion when riding in winter.