Well I sat the 840 in my treadle base, and realised that, because of the oil pan and hook drive gear housing beneath the flat bed being very close to its front edge, I would have to butcher the front wooden lift up flap thing on the treadle cabinet top in order to make the machine sit in the base properly...
... so instead I have attached a 90W generic motor onto the machine, sat it back in the original wooden base it came in, and the machine is running perfectly
I originally intended to use this machine to sew medium-weight leather using a Bernina leather wheel foot, using it in a treadle base, and, (unlike my other vintage Berninas), with no expensive/irreplaceable Bernina motor at risk of blowing out.
But I'll run the machine with the generic motor for the time being at least. If that motor blows out it is no real issue as they are easily replaceable and not too expensive. I also have the option to run the 840 with a more powerful, industrial motor, but I'd need a cabinet or table for that, and at the moment I don't have the space.
I'm a bit disappointed at not being able to treadle the 840, but that does not detract from the fact that I am overjoyed that the sad, forgotten, rusty, seized-solid machine that sat in a garage for who knows how long and cost me £5, is sewing beautifully again!