I found one in a field last year in the grass and looking closer I thought it was AMM at first as it was so dark on the body,I was thinking OOH! has a beekeeper got AMM near me but then noticed the shape was all wrong at the back.
Like Firegazer the one I found was a good 20mm in size.
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.