Black fly in Walthamstow

I have been bitten by black fly several times in the last three years in my back garden in Walthamstow. Two weeks back I was bitten while leading a bike ride at the Waterworks roundabout nearby on the edge of Epping forest. I don't know which genus or species, I only met black fly in the past in the US - and as a student I was told we don't really get them in the UK. (I did work on sub fossil caddis larvae, I've got some knowledge of insects.) Much bigger than the standard Scottish midge.
How do I start on identification? Is there a key I can use without buying a monograph? I'm a member of Buglife. My Ent Soc membership lapsed decades back.