"Rod" wrote in message
...
Doctor Drivel wrote:

"Heliotrope Smith" wrote in message
...

Iv'e always considered microbore to be 6mm or less.

Correct. Mini-bore above 6mm and small-bore using 15mm.

You are at least out of line with some suppliers:

"Microbore Copper Tube 10mm × 25m" - Screwfix

And, in my experience, anyone with CH utilising 15mm pipe calls it
microbore.

Can you refer to an authoritative document for your definition?

Yes, me.