The problem - 1 bathroom and 2 en-suites.

There is one central fan extractor, serving all 3 - all ducted in.

Neither en-suite has an exterior window, so these rooms require the
fan to have a 15 minute run-on.

(I'm planning to use a faceplate switch outside the bathroom/en-suite
door, rather than a pull-cord)

The obvious way to wire it is for the light switch to each room to be
dual-pole, and the second contact from each light switch is commoned
together to the run-on timer (I don't mind if the bathroom goes
through the run-on timer, but it could probably just connect direct to
the fan).

The only dual-pole switch I can find are in the grid-switch range, and
rated at 20A (I'm using MK logic plus range elsewhere).

So I *can* do it.

But is there an easier way using standard switch/faceplate parts?

(and it would be nice if the faceplate switch had a switched indicator
neon so you know the light is switched on inside the bathroom/en-
suite)

Also - as this is a slightly unusual configuration, and the 3 rooms
are 2 downstairs and widely spaced, and 1 upstairs - I'm thinking of
putting it all on its own 6A RCBO marked "wc's & fan".

There will of course be a fan isolator switch next to the fan (which
is built into a studwork cavity in the bathroom wall)