Hopefully fixed it, although, I will have to wait and see if it happens again to prove the point.
Long story short, after checking the glow plugs, several connectors, the relay, and then finally finding where the fuse is, it turned out to be a blown fuse. By the way, the fuse is not in any of the fuse boxes, it is in a connector, on the back of the power steering reservoir bracket.
The problem is, it is a "Special" fuse, which is very similar to a standard cheap 80amp (PAL) block fuse stocked by most motor factors, but just different enough, that you can't fit the cheap alternative in place of the Nissan special one. I did a search online, but no luck, so I guess it's a Nissan main stealer part.
I popped round to the Motor Factor, and picked up 3 of the cheap (82p each) 80amp slow blow fuses, and using the blown Nissan Fuse, hot glue gun, and plenty of heat shrink tubing, I made up an adaptor.
It means I can now look for the proper fuse at a reasonable price, but have a ready supply of cheap fuses to hand, while I get to the bottom of the problem.
I have a feeling it may blow again, as while testing things, the first time I checked the glow plugs, the ammeter briefly showed a reading of 150amp, then dropped back to 54amp, so it looks like one of the glow plugs is going short circuit, I just can't work out which one, as it would not repeat it no matter how much I tried, even after putting them in the freezer, to make them really cold, just in case it was a heat related issue.
I might just order up 4 new ones, and replace the lot as the ones in there are now about 3 years old.
Just a little note to those less electrically savvy, when playing with heavy current devices on a car, like glowplugs, starter motor, head/spot lights etc, never trust a voltmeter, especially a digital voltmeter, to tell you the whole truth
You can go round unplugging things and measuring voltages all over the place, happily getting readings, but still find when you plug it all back together, nothing still works. This is because the amount of current drawn by the digital voltmeter is so low, that even a drip of water over the gap in a blown fuse passes sufficient current for the meter to measure a voltage. I know about this, but it still took me a while to work out why I had 12v at the connector to the glow plugs, but as soon as I plugged it in, the voltage was gone...
Not good Clive 
