Custom libraries and dependency injection
OK people, I played around with a couple of options, but the bottom line is, if you want to create your own 'services' class, specifically for dependancy injection, it has to be tied to a module.

So - I created a custom module.  The actual module file is empty

&lt;?php


Also created the services definition / config file my_module.services.yml and then critically, your services class files should go in a sub-folder /src.  Since it is a service, you will need an interface and implementation class.