Discovery 1 300tdi No preheat :(

Right then I expect that this has been asked and I may be searching for the answer incorrectly so if so I apologise in advance folks.

"Blue" my "R" reg Discovery 300tdi does not seem to have any preheat at all going on. There is no light ever on the dashboard and it was pretty damned sluggish this afternoon when I started it. I know it may be the battery on the way out for the sluggish start in this nippy weather we are having but I have changed the preheat relay and this made no difference at all.

I am open to suggestions here as to what to look for and try to rectify the problem so thanks in advance for the Advice/Mick taking/General laughter!

Have you tried measuring the voltage on number 1 glow plug (its the last plug at the end of the cable). if you get 11 ish volts when you turn the ignition on it means the relay is fine and you may have a duff bulb in the dash. If you don't get any power there should be a 60 amp fuse for it somewhere (don't know where on a disco). Also worth disconnecting the wire from each glow plug and measuring its resistance to earth, should be about 0.2 ohms.

Thanks Kwakerman I will give that a shot on Monday