Which is my master socket?

[#1]

If the house is that old, then you probably have equally peculiar mains routing which will do nothing for the performance of your powerline adapters.

My house is fairly modern, but has been extended and has two electricity distribution boards One connected to the other. With powerline adapters on the same ring main I get 300Mbs between them. Put them on separate rings mains  supplied from the two different boards and  speeds drop to 38Mbs.

What sped do you get with a wired connection directly into the

[#2]

Just checked my broadband speed again, using both the Plusnet speed check and Broadband Speedchecker. Both show a downstream speed of 40Mbps over Ethernet 16Mbps over wireless.

Looks like it was just a question of waiting until the exchange got its act together.