Wouldn't mind testing a pair of PLA's other than the TP-LINK 200Mbps (TL-PA211) but have a feeling that even with a different make or model the interference will still show up.
The completely spare adapters that I have are branded "Simpler Networks", marked on the back as "200Mbps PLC Mini Passthrough". Reportedly these were supplied by BT (or O/R) on some sorts of installs.
I'd be interested in testing these myself as well, do you have any more details on the methodology of your previous testing? However I only have 20CN ADSL.