Might have a resolution on mine here.
Borrowed a clip on ammeter from me dear old Dad. I was a bit surprised to see it was only pulling 37A on preheat, which seemed a little low.
Took all five glow plugs out. Bear in mind these are new Densos, which were recommended to me by a garage as 'never going wrong'. Tested them individually with an ammeter before they were fitted, they all pulled about 10-11A each and lit cherry red on the tips just fine. From install, they worked for just one bloody day before the glow plug not available warning came back on. I have detailed all the testing and checking in this thread already.
Now, one has gone open circuit, one lights up half way down the tip rather than at the end. I am now back to the original 4 working Berus and one Denso - no warning lights now showing!
This is bizarre!
Make something idiot proof and someone will only go and build a better idiot...