There are masses of them on the ivy flowers in the autumn where I sometimes go for a lunchtime walk escaping from my work. I suspect that the quarries nearby are where the maggots live. The adults seem to come in two colour forms, mimicking what you find in native honeybees - an all-brown type and the one with orange spots too.
I've known quite expert beekeepers be fooled by their similarity to honeybees. Once you go back for another look you'll see the difference though.
G.