I have bought a CO detector, only to find that the installation instructions say not to install it in a bathroom. My gas boiler is in a downstairs shower room that includes a shower, toilet and washing machine. Yet I am advised by my gas provider to install a CO detector therein. What is your advice, should I or should I not install a detector in this room?

The reason for not to install detectors in or near bathrooms is because of the likelihood that steam will cause false alarms. I can see no reason why you shouldn't install it in this room, close to the gas boiler away from the shower/washing machine and monitor it to see if it does cause false alarms. Also you could consider power ventilation controlled by the electric light so when this room is in use the level of steam will be reduced, also any harmful gases will be expelled.