Dave - why don't you have a cheap solar panel from Maplins in the cuddy to trickle charge it in the daytime?
I am not 100% but even leisure batteries dont like being totally discharged (something like 80% max).
If you know anyone with a Halfords trade card, they are worth a look, the discount can be 40% on batteries!
I got my last battery from Motorist Discount Centre near Winton\Wallisdown they have a few branches:
http://www.mdc-auto.co.uk/
I wanted a specific Varta Traction battery as I use mine for cranking and electronics. MDC were cheaper than all online and anywhere else, they ordered it in specially for me.
HTH
Rob