All was well until we noticed low battery - yet we are on shore power (finally rigged up the connection to plug into 50A dockside and take on board 230V 5-Hz - close enough to U.K. 240V/60Hz that all the equipment works) BUT then.... the temperature alarm on the battery charger comes on! Disconnected shore power and started the engine to charge batteries. No hot water. Engine alarm sounds (only in the cockpit)!! Low on coolant - included anti-freeze in top-up. It all happens at once! Back to the batteries - inspection : oh dear - they are hot, deformed and clearly no good anymore. Conclusion - will have to buy new ones - no choice. They must have been damaged by repeated discharge to low voltage when we are not on the boat – some boat equipment / instrumentation is draining the charge although the service battery switches are turned off when leaving. Shame - an expensive discovery! What a day!