Al Deveron wrote:

Advice needed...

I have this bathroom light switch which has three connections: S1, S2
and COM.

[...]

The cable from the mains has a red a black and a bare copper earth.
The cable to the light is the same type.

What is the correct way to wire up the switch?

You're saying that the mains feed comes into the switch first? In that
case you want

- Red (L) feed to COM
- Red (L) light to either S1 or S2

It doesn't actually matter which way round the reds go, so long as one
goes to COM and the other to S1 or S2. Then

- 2 x Black (N) connected together in a small terminal block or an
in-line insulated crimp connector; ensure that this cannot short to L or
E; insulate with heatshrink sleeving if necessary

- 2 x Earth sleeved in green/yellow and connected together in the earth
terminal provided in the mounting pattress (or with a terminal block or
in-line crimp if no earth terminal is present).

I assume the wiring is existing; if new you should be using the new
colours where L is brown and N is blue.

--
Andy