I made some enquiries with Clyde Outboards regarding my electrical dilemma.
Guy I spoke to was brand new and completely honest with me when I described my needs.
He turned down the opportunity to sell me an outboard charging kit to allow me to charge my battery in my Sib from the outboard.
He said it was not the best option for my needs.
Instead he recommended utilising my current batteries and purchasing an XS power pack to recharge my plotter, hand held VHF and run my fish finder over my away days.
They XS pack cost around 100 squid and I can think of other applications I could use them in - small compact and even capable of jump starting a large 4x4. It comes with a host of leads etc and it can fit in your pocket.
Apparently they are quickly becoming must have items amongst the boating / camping community in and around Loch Lomond.