@Adam - well, the Canon EF 50mm f/1.8 STM lens arrived today, and it's a beauty. Plastic, yes, but extremely solid and very nicely finished for the price. Much more solid and refined than the Yongnuo offerings (and the old Canon 50mm f/1.8 also, I understand). The only weird thing is the stepper motor "focus by wire" when in MF, or when overriding AF (a la Pentax Quick-Shift) - but it seems to work well enough. Fast and quiet AF too. I've had it fitted to the EOS 5 all day and have played around with it quite a bit, and I still have a full battery according to the indicator on the camera's LCD panel. You wouldn't *believe* how relieved I am... In my heart, I knew the problem was with those Yongnuo lenses, but the way my luck's been going of late, I had an awful feeling that I'd see the same battery drain problem with the Canon lens, and it would turn out to be a fault with my beautiful EOS 5
Anyway, thanks again for the advice. Now I just need to get out and shoot this baby!