OP, unfortunately you're the one that's been to site and none of us (to my knowledge) are telepathic (although I am getting hints of beer and vino emanating from around Cheshire/Staffordshire and Northants for some reason) so it's natural we're going to ask questions to try and establish the fullest picture we can before dishing out advice, hence you see it happen a lot.
It's not meant to be a pop quiz, but your OP was a bit thin in terms of information especially regarding the measurements. 60v.... between where? (you've answered, stating between L-N and L-E), but when measured at the board (I'm assuming the neutral/earth bar and the MCB output terminal screwhead) you're getting 240v.
So if this was my problem, I'd be checking terminations and device connections in the CU. Especially as the issue appears to be affecting all sockets on the circuit... if it's a ring final circuit there is only one common place and that would be the CU, unless it's a lollipop, then there are two. If it's a radial, are the sockets the only thing on the circuit? If there is maybe a SFCU as the first connection, could be a loose connection in there which is having a knock on effect on the rest of them. But, as you can see, there are a lot of possibilities and so lots of questions.
Do you have other electrical test equipment (such as an earth fault loop impedance meter, low range continuity tester etc.) to test the circuit itself for other faults?
IEC cables to USA plugs should have been clue ... GONE !
Where did that come from?
