Despite getting an email from Zen last week telling me my upgrade was on hold, Openreach have just completed it. It was a bit of a cluster duck as the engineer was given the main Zen phone number as a contact. He called them and was told they would contact me and let me know that it was going ahead. First I knew was when my line went dead, but as I can see the cab from my window so just asked the engineer if he was working on my line. 30 minutes later he'd been in and changed the faceplate and job was complete.
The speeds are a bit up and down at the moment, but seem to be settling down at the top end of expected.
Good job Openreach, not so good Zen.