I hate to do this, particularly so quickly after posting but I found the issue and of course it all makes sense now.
In my app.php I had done the following:
$app->register(App\Providers\AppServiceProvider::class);
$app->register(Illuminate\Mail\MailServiceProvider::class);
$app->configure('services');
But I had not done the following hence the error about driver being NULL:
$app->configure('mail');
Hopefully this will be helpful information for somebody else down the road!