Seems that there was some spurious negative values on the watt readings when it came back online. Although data is feeding nicely into Home Assistant, I’m now unable to ping or access the web gui of IoTaWatt itself - any further ideas (have tried an off/on of the device to no avail).
EDIT: after searching around the forums I found this and it seems the Home Assistant Integration is to blame. Disabled and GUI works again. Still unsure on the spurious values but can deal with that now I can access the GUI.