In case this happens to someone else I’ll detail the solution.
OpenEnergyMonitor kindly checked out the hardware they had sold me and pronounced it OK.
When I reinstalled I had the same problem but on a different input.
I then started a new node on Emoncms.org and moved each input over to it, using identical Process Lists to create new feeds. Have had no problems since.
Can only conclude that the problem was within the original node on the Emoncms.org site.