I finally got the CD changer all mounted in and up and running, after some hassle with the original radio. I won't say what the problem was as it's a bit embarrassing, but I probably should go to specsavers! Many thanks John for your advice and not laughing at my little error, the installation went well and it is all up and running perfectly.
With car out on the drive I finally had chance to look at the front fog light problem. The relay fitted is a normal 4 terminal non-latching relay, poss and neg for coil and contacts in and out. On checking the switch assembly I realised that the front fog lamp switch is indeed of the latching type, but mine is worn, as it will only occasionally latch if you lightly jab the button.
A quick look for the combined front and rear fog switch YUG101420 reveals Rimmers have none and there don't appear to be any on e-bay at the minute. I have stripped a rear fog only one I had to see what's inside and found that behind the button is a miniature, non-latching microswitch soldered to a pcb. Hopefully if I can get the front fog switch assembly apart I should find a similar microswitch, but of the latching variety. I can then try to find a suitable replacement and put it all back together. The difficult part is you have to remove the individual button covers to allow the pcb and switch assembly to be removed from the rear of the switch holder. If it all goes according to plan I'll try to put some pictures on and a bit of a guide, for anyone else who has the same problem.