Well well well....If I'd known it was going to cause such controversy
I would have thought of a different way to ask the question!!

Let everyone be assured that the house won't fall down because it has
a straight floor and by the time I'm finished it will be stronger than
it was to start with. That, of course, wasn't the question: I was
simply after some suggestions for how I might do a decent job of
cutting the top straight!

I might have to do some off site experiments and see what 'feels' best
between Jim K's power plane, Ghostrecon's router and dennis's guided
saw, though I suspect that whatever I use will be a bit of a bugger
all the way along a joist. I would replace it as suggested but that
would mean I'd have to rewire half the house as pretty much every
cable in the house goes through this joist.

Cheers people