Thanks for all your comments..
Robert: The Midi looks great, but I think it is too far a hop for me at my level of experience, and a bit too much Biscay..
I think we are planning the shortest Dover >Calais trip that we can, on a good day, with careful attention to shipping lanes and AIS. Might have the confidence to do the Midi route on the way back with a years more experience under my belt.
Johnathan; Yes minimising the overhang would be advantageous. I am a little wary of getting knocked around by bigger boats in the canals.
Robert: The six tyres advice sounds sensible. I was even thinking of lashing 2x 8ft scaffolding poles across the deck fore and aft, and attaching the tyres to each end. That way if a really heavy boat tries to crush us we will have some protection..