Guys, firstly, Thank you. You have the patience of saints!
Believe it or not, I am actually a Software developer/Electronics engineer by trade but PC based. The Arduino platform is a new experience for me and my previous uC experience is 20years old!

I now have a working mqttClientGateway. I need to play some more - but first indications are that it is working well.

My problems were a combination of bad folder structure and also bad combinations of sketches and libraries (desperately trying to play down the lack of experience factor here!!). Once I got this sorted, it started working.

A huge 'High 5' to you guys. I owe you a bier or maybe 2!

Not a question for this thread I know, but before I leave you in peace - what are you using for your Home Automation controllers? I have been playing with openHab and this seems to work well, but I have not really investigated the others.