Pete, I would have a look at the Odyssey then. Same technology as the original (AGM) but extremely durable, powerful and... heavy! ;D
I had one on all my flat twins (Old airhead have weak charging system and very demanding Bosch starters) and never had a problem.
One of my Odyssey had its voltage going down a bit after 3 years. I contacted the manufacturer to try to have an explanation, and they shipped me a new one at no charge. A couple of months later, I realized that my voltage regulator was doing free games and putting very low (7 volts) or very high (17 volts) tension on the battery. :-[