Power line adaptors - checking power sockets?

[#1]

I think Oldjim in reply 2 was on the right path
I have a loft conversion where one socket will work with home plugs and another one won't. Reason is that a new circuit was put in for the conversion and the plug that works is on the old circuit connected to the downstairs sockets. You need to check the cbs in the distribution unit or units. 

[#2]

I have successfully run a pair of TP-Link 200mbps home plugs on across two ring mains connected to different fuse boxes. The crucial thing is both fuse boxes were on the same meter (consumer unit).
If it was an issue with it being different ring mains etc I'd still expect it to work but perhaps nowhere near full speed.

[#3]

I'm not sure but it could make a difference if each ring / spur was on a different RCD (not sure what effect these type of devices have an RF signals on the wires).

[#4]

If the wiring is fairly old (40+ years), it could even be Aluminium - in which case I'd just switch everything off at the mains. 

[#5]

artfulbodger,
What I have noticed on TP powerline adapters is they don't always pair if to far away.
This is the fix to try.
Put 2 power line adapters immediately next to each other (so 2 plugged into wall next to each other), then do the regular pairing procedure by pressing the pair buttons on each unit.
The pairing information is then save inside each powerline adapter, I can only assume there is a rechargeable battery in them.
Then unplug them and install in the sockets you originally intended. 
Because of the pairing data that's just been saved there is a chance they will start working.