The two reds should be in one terminal point on the switch, the COM
terminal. The black should actually be covered with a red coloured sleeving
to mark it as a switched live to the light fitting on the ceiling. The
setup you have is one red is from the mains and is the live feed to both the
switch and also the continuous feed for the fan. The black from the fan
will be connected to the black inside the ceiling rose of the light fitting
along with a switched red to make the timer in the fan come on when the
light does.

Thanks for this info, but I am still a little unsure of how to
proceed. Am I right that a normal light switch (with one COM and one
L1 terminal) will not be usable?

If I need another type of switch (a normal switch with four terminals
is what the other bathroom has) can you tell me what this is called,
and which wires go where?

Thanks very much again for your reply. Cheers Nick