Thank you for your reply and apologies for not responding sooner - I didn't get a notification of your message being posted.
I think the hub may well be one of the new compatible ones as I was able to connect to it and the troubleshooter gave both fttp & regular broadband options.
My Dad has given an least one (maybe several) of the BT staff he has spoken to a number (which I assume is the ONT serial number you refer to) as the builder told him that BT would need it. I did try connecting both the hub and phone to the box but neither worked.
I have just tried dialing the old number and it rings so looks like it hasn't been disconnected. My Dad's BT account is still showing the old number too.
The really frustrating thing is that no-one at BT seems to want to sort it out or can tell him what the situation is. We could understand if they had told him when he initially made the call that it would take x weeks or whatever, but to keep fobbing him off with "someone will call you back" and then never doing so is appalling.
They are away for Christmas now for a few days so we're hoping that when he calls when they get back something might get sorted out.