Connecting my glowplugs

Evening all, I've got a disco 300 in my 1989 90 and although i've never needed the glow plugs in 3 years, i'd like to connect them up for peace of mind. All the cables are linked together but i don't have the nut that holds the bulkhead cable to the first plug, is it a particular sized nut i can buy or is there a specific part number?

Also, having never used/needed them, how can i tell if they are all working correctly?

Many thanks, Harry

4mm standard nut ,in mean time take nut off no1 and put on no4 at least 3 will work

Thanks mate, had a look at a 200tdi -they look similar and my mate has one spare so will try that. if not i'll pop down the ag. suppliers and get a nut

Harry