Does anyone have any suggestions on how to measure the DC current taken or detect if it outputs thermal noise when pointed at the ground, wet rag etc:

The specification is here: https://www.idirect.net/~/media/Files/Spec%20Sheets/SPEC-SHEET-Uni-Ku-PLL-LNB-0115.pdf

If your LNB has an external manual switch you need to make sure it is set to the correct band (high or low).

If you have renewed the cables, then the centre pin should stick out about 1.5mm above the rim of the F connector. The outer braid must make good contact with the outer of the plug and no small braid fragments nuts short circuit the inner to the outer.  The DC power from the indoor unit must get to the outdoor unit. The current should be 145 to 170 mA.  I don't know if this figure is displayed on the modem screen.

There are some more ideas here: https://www.linksystems-uk.com/connection-troubleshooting/

best regards, Eric