Kevin, Thanks for replying. I'll try all that you have said

Thanks for that. I wonder if yours works ok because CU b appears to be a spur from the other unit. Sadly my two consumer units are separate albeit they are supplied by the same power source.

Any advice please? I am using 3 powerline extenders in my home however part of my house had a former granny annex which has its own consumer unit and therefore as the wiring circuits are different I...