After another series of emails with Gary (I hope he's not getting sick of me!) I've settled on six meters overall length. Hopefully this will mean I can still fit my car in the garage over winter while I work on the boat.
To maximise the buoyancy gained Gary recommended adding the extra 600mm to the middle of the boat. I will also be making the bulk heads a bit longer to help in case of a capsize or swamping (as Gary pointed out it will also mean less bailing).
Gary was also kind enough to send me the plans for a larger ama, which will allow me to add a bit of extra sail area to add a trampoline to the cross beams for resting on
I'm also toying with the idea of installing lockers under the seats with a couple of tackle and storage boxes in them. This will give me a small amount of water tight storage for valuables like phone and wallet.