Well that was fun. My dad turned the oven on, the switch went "bang" and emitted sparks, the RCD tripped and everything went dark. Thankfully the (built-in) oven unit has two oven compartments with separate controls and the small top oven seems to work fine, so we can still eat, but it's gonna be fun getting the whole oven unit out and fixing the main oven's switch. (I'm guessing the switch itself has failed somehow and shorted to earth, but that remains to be seen.)