Does the isolation switch for a bathroom fan have to be 'out in the
open' as it were or can it just be accessible in the same way as
junction boxes etc. must be accessible?

In other words can I put the isolation switch somewhere that's
accessible if you're going to service the fan but not accessible if
you're just using the bathroom?

Also I believe the isolation switch must be two pole (if at the supply
end of things) or even three pole if it's adjacent to the fan and thus
needs to break the time delay feed as well. Come to think of it a
three pole switch is almost mandatory isn't it, otherwise you just
about have to turn the light off as well.

--
Chris Green )

Come to think of it a three pole switch is almost mandatory
isn't it,

Yes. 3 pole isolators for fans are widely available.

Christian.