After considering various options, I chose a Halfords standard HB063 12 volt car battery. This battery requires no maintenance, apart from charging. I have placed it amidships within the cabin, where it makes good ballast. The GPS units can now be powered by it, as I have linked them via a three-way cigarette adaptor. The Autohelm plugs into an extension socket that is wired to the battery.
At the moment I am reliant upon charging the battery through a mains connection, but I may buy a solar panel charger for topping it up when I am cruising for more than a couple of days without an opportunity for plugging into a mains power source.
Very shortly ‘Sandpiper’ will be ready for her first sail with me, and I am hoping for a fine spell of weather next week when I can try her out. Two or three days messing about on the River Crouch should give me an idea as to how she handles and of her potential for cruising further afield.