Now the conundrum - with the light OFF there's 240V across the swtich.
Turn the light ON and the voltage vanishes.

Any clues?

Yep, you're reading the voltage across the switch, rather than the
voltage between the terminals and neutral - so you'll get the
situation you describe. If you attach one side of your meter to
neutral or earth and measure the terminal voltages again you'll see
230V on both terminals with the switch ON and 230V on one with 0V on
the other with the switch OFF.

Trust me, the voltage hasn't 'vanished'!!

:-)

Dave