Thanks to all for the replies. Sorry I couldn't reply earlier but I'll answer your questions as best I can.
I don't think it's relevant to the 'problem' but the boat is 17 years old, three cyclinder Beta, and domestic generator was changed a couple years ago to [I think] 90A or 120A.
The external controller is a Pro Reg D (the one with a fan on the top) and looking at the picture on the Sterling website I can confirm there are no temperature sensors connected.
The battery charger is a Victron Blue Power IP22 12V/30A Three outputs.
New batteries and battery charger were installed last May.
Low load demand. Shorepower 12V fridge, Avtex low power tv, LED lights + usual pumps.
I take on board that I might need to drop the voltage from 14.8V to 14.4V. I'll take a look at the battery manufacturer's spec when I next visit the boat.
As the batteries are as fully charged as they're likely to get, the current drawn at the 15.3V is negligible.
I don't think the situation is temperature related because I first noticed it last June on the first trip out after the batteries/charger were installed. It was certainly not a cold day and I arrived at the marina, loaded up, disconnected the mains, then immediately set sail. I noticed the high voltage about 10 minutes later while waiting at our first lock. The trip was 18 days (Calcutt to Oxford return) and the high voltage didn't appear again throughout the trip.
Second trip was the same, (21 days September, Calcutt-Stratford return), high voltage on the first day after disconnecting the mains then everything OK thereafter.
Third trip was the same (late October 7 days locally).
I don't know how long the high voltage existed but on the second trip we'd been cruising for nearly half an hour before I picked it up, so it appears not to be a short-lived event. But once the controller drops back into float mode the voltage seems to correct itself.
We don't normally spend nights in the marina but last week we decided to go up for 3 nights, relaxing to get over the effects of having the house full of grand-kids over Xmas. That's where I noticed the high voltage situation again, a couple minutes after turning on the engine for the first time to get some hot water (note that the controller has a 2 minute soft start feature).
So, this is when I turned off the engine and ran on batteries for a couple hours to take the edge off the battery voltage before starting up again, whereupon everything went back to normal. I spent the three days running from battery with the charger turned off and, once again, the situation didn't reappear.
So I don't think there's much doubt that temperature is not a factor, but the mains charger is! Perhaps there is reservoir capacitor or something similar on the charger's output that remains charged for a period after the charger is turned off which somehow affects the controller.
On reflection, what I should have done was to have turned the charger back on for a short period, say 5, 10, 20, 30 minutes.... between engine starts, to see when the problem returned, but I was too 'relaxed' to bother about it by then :)!
Dr Bob: I'd be interested to hear if you experience the same results on your next trip to the pump-out. I wonder if anyone else has noticed similar symtoms.