I would prefer a better source than Amazon but it appears that corrugated pipe is specified in the old British way ie by internal diameter. The following is an example from:
https://www.amazon.co.uk/Corrugated-Stainless-Steel-Pipe-DN25/dp/B07B2JVJFX
Nominal width: DN25
Wall thickness: 0.20 mm
Inner diameter: 25.5 mm
Outer diameter: 31.8 mm
Bending radius: 39 mm
Operating pressure: 10 bar
This appears to be compatible with my specification to Telford and my 1 inch (28mm) pipes. What was not clear to me was the use of corrugated pipes to replace my original standard coil.
I had hoped that somebody would say they had some experience of the use of corrugated coils in a gravity system.
I am trying to compile a full list of possible reasons for the replacement cylinder not getting any heat. Here is my list so far:
1 - Partial or total blockage which allows full heat to bathroom radiator but no heat to cylinder.
2 - Incorrect or imprudent connections to cylinder.
3 - Partial blockage inside cylinder which still allows it to be flushed.
4 - New cylinder fails to self-clear air locks like all earlier cylinders did.
5 - Corrugations inhibit upward air flow in the face of static or slow downward water flow.
6 - Slight slopes on 'horizontal' pipes more critical than previously.
I suspect that if the cylinder fails to self-clear air locks then it will be a continuing source of trouble in the future. My experience is that large old vented systems can slowly suck in small quantities of air at any time.