Trevor at Adverc recommended a larger alternator and some Absorbed Glass Mat (AGM) batteries. This type have a longer cycle life and are low maintenance. I am told that the new spec with 600 Amp Hours of battery capacity and a 110 Amp alternator should meet our needs. Only time will tell as the proof is in the pudding. My old batteries were showing their age after 3 ½ years having lost much of their capacity. One of the reasons given for the 70 Amp alternators failing was that it did not have enough output for the loads put apon it.
AGM battery bank
Adverc delivered the goodies to Great Haven Marine Services and John installed it all. We were so lucky to be able to get in and out of the marina in between periods of ice and snow. Got to a mooring by 4 pm travelling through a blizzard! The batteries were full by then and we did not need to run the engine that evening as was previously usual. The next day after 18 hours of use the batteries were still reading 12.3 Volts after supplying 67 Amp Hours of power.