Thanks, folks,
You have given me a lot to think about here.
The single belt drives the water pump and the single alternator (60A, I think).
I have a bank of 1 x 90AH cranking battery and 2 x 110AH domestic batteries. The cranking battery is about 2.5 years old, but the domestic batteries are about 5 years old and are about shot. As I am hardly using the boat at the moment, due to working abroad, I do not want to replace the domestic batteries for them to just deteriorate.
I haven't had time to check the pulley alignment, or for play in the alternator or water pump bearings, but I will do this soon.
I think I might disconnect the domestic batteries altogether and see how things go.
Thanks again,
Chris