Thank you for your interest and help.
The engine is a Vetus 33hp. The make of alternator isn't listed in the manual but I'll try and look when the rain stops. Likewise for pulley sizes, but you may not need this info because I adjust the engine revs when charging to get 14.8 volts, for 1-1.5 hours [usually but variable, average daily total 4-5 hours] The fridge is 12v, no 240v or gas option, and it makes a very quiet buzz rather than a hum! When the smartguage was fitted all the main cables were replaced with thicker ones 13mm diameter including the rubber.
During seven years of boating much advice about batteries, and work, has been conflicting and/or not specific enough, so the help from yourself and other posters has been a breath of fresh air. Of all the work done, the most useful has been the fitting of the charge controller - it has clearly given me much better charging for the same amount of engine running.
The electric blanket consumes 10ah per hour and is rarely on for more than 1 hour. The real biggie would seem to be the fridge. A friend who is technically savvy looked for alternators on Ebay. He found an Iskra 175 which is exactly what nickhlx suggested. It didn't sell so might be what I need.