Best Beloved’s web diligence and some subsequent google-fu has revealed a likely culprit for the bites: Blandford Fly (not the beer). Looking at that and other / linksshows a lot of matches. Small, black flies 2-3mm in length? Check (there were a lot them about). Oxfordshire? Check. Bites predominantly, indeed exclusively on lower legs? Check. Above all the descriptions of the bite sound about right, apart from the fact that I didn’t feel anything until it was too late. Maybe weeding is such an itchy process anyway that I just tuned it out. Hmm: not disease carriers (good) but secondary infections are possible (bad) …
For what it’s worth the red patches are consolidating, shrinking a little and turning more purple. I spent most of yesterday tired and cold and was in bed by 9, which was probably a reaction to several gallons of toxin in my bloodstream but not quite enough to warrant taking time off work.
Apparently the biters would all have been female, requiring a blood meal before or after mating to produce hundreds of eggs. I’ve never before been swarmed by sex-crazed females and it’s a shame that when it finally happens they all turn out to be insects, but I’m glad I could perform a service.