In case this happens to anyone else, whilst on my bulb swapping expedition I found that my rear offside outer light cluster had lost one nut and was about to lose another! No idea how after all these years but luckily I found it.
Anyway looking at the surviving nut I thought that looks familiar.... I checked my my glow plugs and voila! Same nut
Anyway looking at the surviving nut I thought that looks familiar.... I checked my my glow plugs and voila! Same nut