Yeah - there were a couple of warning triangles that alerted me of issues that needed resolving.
I completed all the issues and tried to publish an internal test. Nothing. Still wouldn't connect to my phone.
hen got an email about 6.30 last night telling me my game had been published - panicked as i thought this might mean it was published live on the Play Store - checked and it wasn't there. Phew. But checked the game on my phone and the leaderboard was now connecting(!) - seems there's a time delay between publishing it even internally and it propagating through.
Still not completley sure about how I've managed to get it to connect, only that it does now.
There are things in the App Signing tabs called Upload Certificates and App Signing certificates - not sure if i need these or what they're for - I signed my app on my Mac before build etc.