Hi all - thanks for all the tips, very helpful indeed!
In the end I found the present belt to be excessively worn again, so I also checked the pulley wheel
for rust as well (entirely possible as I do have a very old wooden boat!) and found there was a fair bit of rust and some shiny patches. I then ran the engine
without alternator belt and ran a fine file in the pulley wheel
which seems to have given me a good surface and still plenty of space for a new belt, which I have put on.
I suspect I have a standard alternator that comes with this engine, but I am fairly sure it wasn't the battery
bank as they are brand new batteries (although heavily used as I only have space for two totalling 110 AH).
Using a ruler, the alternator pulley seemed to be very slightly out of alignment, maybe one or two degrees. I'm not sure how much this would have affected the wear, but I am also not sure how to correct it as the alternator bracket appears to be a casting.
I will only know if I've solved
the problem in the next 4-5 running hours, but looking good early on.
Thanks again all.