I used a 25A fuse between my leisure battery and fuse box. It needs to be sufficient to cover the maximum anticipated power draw of all your devices. The cable rating must also be higher than the fuse rating (so the cable doesn't heat up and melt before the fuse blows). You should also place the fuse as close to the positive terminal as possible, to minimise unfused cable run.
Fuses between leisure and starter battery positives and split charger were 40A. Again cable rating must be well above that.
The rating of your battery in amp hours has nothing to do with your choice of fuses (that is just it's capacity).
I ordered a custom split charge kit from 12V Planet. They take care of cable capacities, fuse ratings etc, and make it up for you with your desired cable run lengths. I found them very helpful.
http://www.12voltplanet.co.uk/split-cha ... asure.html