This is a good'un!!!!

[danspark12345]

So.

Went to test RCD with MFT at the socket. My meter alarmed and said reverse polarity. I then checked the connections, they were correct.

I then put my little plug in tester in and it showed something was wrong but doesn't say what. (Look at picture).

Baffled. I then check polarity at the board and the meter. All good.

I plugged my plug in tester into all sockets in the house. I had the same warning in every socket. This tells me the problem is upstream. But everything seems fine

 

[old_guy]

Is it possible all sockets have LN switched at the terminals?

 

[Rockingit]

I'm rashly assuming that you've checked the incoming tails at the CU aren't reversed?