It's just jargon meaning the blue (or black if old wiring!) neutral wire has become disconnected somewhere on its way from the supply to the transformer in the ceiling
Wohooo they're working.
As I said in my previous post, I tried the new halogen bulbs first, unfortunately that didn't work.
Then I changed the transformer form the halogen transformer to the new LED supported transformer, then fitted the new LED MR11 bulbs and thankfully they worked.
I guess we can conclude that the halogen transformer was defective.
Now I've got the job of putting my floating ceiling back together, a job I learned I didn't need to do, as seen in the picture, it turns out the transformer was hidden above a hole in the original ceiling plasterboard between the loft insulation, I should have lifted that up when I first got in the loft. It's a bit of a chore putting it all back, but I'll do it with the biggest smile on my face.
Thanks to everybody who responded, I could not have done it without your help, I bow to your expertise.
I've leaned a few things, didn't know about transformers and the different types. It was a bit daunting, but it goes to show, you can achieve anything with a bit of patience, confidence and some good advice.
Thanks again very much appreciated.
David