Thank you all very much for your help.
I'm not sure why it is fitted as a two way switch, as I haven't been able to find another switch in the flat which operates that light. So I think it was previously wired as a one way switch.
The diagrams are very helpful - thanks for pointing me in...
I am replacing a two way pull switch for a bathroom light which had worn out.
I have three red wires which are otherwise unmarked:
wire A (live) was in one connection;
wire B had come out of its connection by the time I could see it, but I from its position I am pretty sure it was in the...