Quick question - shower isolator switch is broken - is this something I can replace (with the help of my trusty Readers Digest DIY manual), or should I get an electrician in to do it?
Hmmmmmmm. From experience, most dangerous electrical faults (undetected by RCD and/or MCB) are caused by bad quality of connections and wiring positioning, as opposed to an insufficent knowledge of circuitry.
Considering the high-load item and it's human safety risk I would advise you to go for the electrician route.
Buy the switch and you may get a sparky on their way home to knock it out for €20.