With the recent postings about batteries, I had a little peek on the net for info about marine batteries and this is what I got.
http://www.lifelinebatteries.com/
http://www.tayna.co.uk/catalog/1186/0/MarineBoat-Batteries-Lifeline-AGM-Batteries-page1.html
bit pricey - but if you get the milage might be worth having a look.
I have heard that marine gel batteries should be good since they cope with excessive vibration better, but it would be an expensive experiment :-\ , especially if it failed !
It would help if they showed the weight of the batteries as well, since the lighter the vehicle the better the range.
My own personal preference is still for car batteries , they are lighter and cheaper and seem to last just as long. I accept not everyone agrees though :)