Hi mate,
It is noisy, particularly as the alternator chain needs tightening, but this noise came on suddenly whilst riding. I know the various noises the bike makes and this definitely isn't one of them!
Good idea with the filler cap - hadn't thought of that one.
Yeah - screwdriver didn't occur to me. The ratchet extension I had nearby worked well enough I tried that trick on the water pump cover as you suggested (and the alternator) and couldn't hear anything out of the ordinary.
Cheers!