Daft question but you have checked with headlights on, if its like Mk2 I'm sure headlights have to be manual switched and not in Auto mode for rear fog to work, never checked mine as never had fog thick enough (some people use them in heavy rain tut tut, clue is in the name, fog light)

 

Hate to be a party pooper, but the Highway Code was amended to allow if driver feels it's needed to use rear Fog lights in heavy rain (100m or less visibility), when motorway spray imitates Fog conditions to reduce the possiblity of a rear end accident.

"Driving in adverse weather conditions (226 to 237)

226
You MUST use headlights when visibility is seriously reduced, generally when you cannot see for more than 100 metres (328 feet). You may also use front or rear fog lights but you MUST switch them off when visibility improves (see Rule 236). Law RVLR regs 25 & 27 "

 

Thanks for the reply, - there is no such thing as a daft question (although I have occasionally seen daft answers). I have tried it with headlights on manual and in the dark on auto. Still not working.

 

Update: you do not have to remove the bumper totally ( but it may be prudent to ) . I managed to get to the unit by removing the lower bumper fixings and all the bumper fixings on the drivers side. You can then reach the back of the fog lamp and 1/4 turn and the bulb holder comes out. In my case the bulb had not blown but was corroded. So now cleaned, thin coat of grease and Now back working. Hopefully will get a pass mot certificate now!