my temp gauge has stopped working, car is running fine though. Have read previous threads, and I guess it is temp housing assembly which needs replacing. Is this a difficult job or just unbolting, replacing with O ring and topping up coolant?

Also, is it urgent job or am I o.k for a few weeks ?

do it asap. the engine could start over heating, an you will never know until it goes bang. trust me it happened to my old favort.

Had recent experience of coolant temp sensor failure, the gauge stayed at the bottom, no movement at all. The coolant temp was fine, the heater got hot quickly etc, there was just no signal to the gauge. If the heater is producing less heat/taking longer and the gauge eventually moves just a bit then maybe the thermostat/housing is more likely at fault. Replacing the cts is a piece of cake if that's the fault. Both items seem to commonly fail.