I think the technical discussions on here have sealed the deal for me chaps.
The rectifier / regulator route sounds I bit too technical (for me).
I'd be bricking it in case my handy work resulted in blowng up my battery, frying the electrics or knackering my outboard.
I think a deep cycle leisure battery will be the order of the day.
I'll make a little storage compartment in the bow box and run a cable to my vhf.
I can charge it in situ when I get home without having to remove it.
Keeping it very simple........
Another point to consider is my outboard is on and off the transom and in and out of my car boot when under tow.
Also adding a cable to the outboard to charge the battery slightly complicates removal / installation - not a major issue tho.
A mechanical failure of the outboard is going to result in a trip being cut short as my auxiliary won't be able to charge up my battery.
I'll carry enough juice in my battery assortment to last my 3-4 day trip or scale back if it's only a day trip.
When I say farewell to the Sib I'll be sure to buy a set up that is ready sorted with capability to charge a battery tho