Thanks for that advice Madskp. I swapped the Masterlink cables between the working and non-working setups and demonstrated that it was the cable at fault. It was two pieces joined by the B&O Masterlink connector. It looked fine inside but after using the B&O IDC tool to press down firmly on each wire, then trying again, it all worked properly as before.
I suppose that one or more of the connections was marginal and that moving the cable to store it while the building work was done must have just loosened one.
