Oh dear.... I suddenly worked out the reason...
Not recently, but over the past few months we've put in place some parental controls on the internet at home.. This takes effect when they boys are back from school. In addition to some mild 'safety' cover in terms of content it also stops any Youtube access. When I was testing it, the parental controls were timed to be 'off'.... I guess that when I was having the issues they were enabled and therefore it couldn't access the Admob / Firebase services... Arghhhhghghgh!!! Sorry!! It's almost laughable if it hadn't taken up so much time and effort and chopping and changing of code, SDKs, Libraries, B4A versions and so on..