I'm afraid I agree entirely with the last post James: It is NOT a testable item on the mot.
However I believe that having a warning device is a legal requirement, (and one that have chosen to ignore on at least 10 cars over the years).

I was told it was, as they plug a device in to test whether it all works as it should down the test centre..?

They do plug a device into your socket but it only tests that it is working correctly..
So whoever told u that James they are talking ball licks..

Not exactly:
As Matt said, this is what is done on the mot:

I assume you have the standard old 7-pin/12N socket, so this test does not apply to your wiring. Even if it did, they test to 'check that the socket is wired to correctly operate the trailer', so this does not include testing to check that the warning light/buzzer is operational, as this is part of the operation of the car, not the trailer.

So, although you are right that it is a legal requirement, it is one of many legal requirements that are not part of the mot.