bathroom light switches on by itself?

From what you describe...

The ceiling light appears to be getting it's power from a source other than the switch -

light coming on when not asked to with two separate switches and the switches having no effect on the light.

An electrician may be the best solution for you particularly in a bathroom.

Peccavi
December 2009

There may be a short circuit in the switch wire, possibly caused by poor insulation.

Use a continuity tester to check if the old switch actually is faulty. If it isn't, turn off the power at the consumer unit, take the switch wires out of the ceiling rose and test the resistance between them with the switch closed, then with it open. You could even disconnect the switch to test just the wire; if there is a low resistance between the wires you may need to replace the cable.

Adam
December 2009