Thank you for your VERY detailed reply and the amount of time you've obviously spent on it. It is very much appreciated.
1) The second second purge line didn't have an extrusion specified because I had assumed it would be inherited from the previous line. Fixed.
2) Re "the hot end goes to 170 for bed leveling": Why? What does that do that my "M104 S{material_print_temperature_layer_0}" doesn't?
3) Intriguing idea re second printer, I'd never thought of that. Done.
I've implemented your suggestions but I'm still getting clicking. The fight goes on.
The settings in my post were after a _lot_ of trial and, mostly, error to see what would work so I'm not surprised at the lack of quality.
Sorry about the delay in responding but by the time I'd posted my query I'd had enough of the printer for one day.