On Tue, 16 Jul 2019 08:47:53 +0100, Adam Funk wrote:

But I also want to fit two 1-gang boxes in case I need networking or
phone wiring later, so I was going to run 20 mm conduit through the
floor to make it easy to draw the cables.

I think you'd be better off just having holes into the void and a
draw string(*) rather than restricting things into 20 mm conduit.
It'll be a damn sight easier to pull things in through just a couple
of holes than through 20 mm conduit, especially once there are other
cables in the conduit. It'll be difficult if not impossible to pull
around even a swept bend in 20 mm conduit.

Or put the cables (Cat5e will do Gigabit ethernet and phone) in now
while you have easy access, cable is cheap even real copper. Don't
get CCS (copper clad steel) or CCA (copper clad aluminium).

(*) 2.25 times the length required with each end securely fixed at
each access point so you don't "accidentally" remove it when pulling
a cable in.

--
Cheers
Dave.