Just as a quick follow-up to this. Relocating the antenna within the reach of it's own 1.5m wire didn't appear to help. I asked Octopus to request a T3 external aerial be fitted but their response was comical:-
"Our engineers have been in touch and notified us that T3 aerials are very new and the process hasn’t been established yet and they are waiting to receive an update on this. We recommend getting in touch over the next few months to see if they're available."
So I purchased a 5m SMA extension cable from Amazon for a tenner, relocated the antenna to high up in the room above the cellar and then rebooted the communications part of the meter.
Not sure if it was a coincidence or not, but I got an email from Octopus that same night saying that it was all up and running now.
So, all good finally!