Sime Friendly Boiler Heating comes on when running bath/shower!?

My Sime Friendly Boiler is coming on whenever someone turns on to take a shower/bath/or do the washing up. The timer button is not switched on and none of the timer "teeth" are pressed in as they would be to set a time for water to heat up. The flat is boiling hot and I am worried about the gas bill - please help! Thank you!