I am not using the smartplug on the powerline plug. Apologies if I gave this impression.
The powerline plugs are plugged into the wall socket. The smart plugs are used elsewhere in the house to control other devices.
I assumed you were using them elsewhere. If they are causing interference it won't matter where they are plugged in, the whole ring main will be affected.
I have exactly the same issue with the same make of smart plugs though my powerline adapters are brand new TP-Link ones. I can buy new smart plugs if I really have to, I can see that there's a TP-Link 4-pack for £30 but I don't want to get exactly the same problem with new plugs, did you end up getting new ones and did they work?
I am using the BT500 Powerline plugs and I also have quite a few TPlink smart plugs scattered around and I am not having any issues.