To those that I met on the 22nd Jan, thanks again for your help.
Arrow Flight is working once again thanks to the skills of Steve from SEA Marine. Steve was the guy who provided telephone help on behalf of Sea Start when we were stuck in Yarmouth with an engine that wouldn't start. He also turned out to my house the following wednesday to have a look at whatever was causing the problem. He'd found it within 10 minutes of arriving and it turned out to be a loose crown nut that secures the flywheel to the crank.
The crank splines had been grinding away at the flywheel splines as the flywheel floated up and down during every start, because of the loose nylock nut. Our assumption is that the nut has been like this since I got the rib three years ago and it has taken that long to grind away the flywheel splines. The nut should have been torqued down and secured with locktite but that has never been done.
The fact that the rib had been serviced 6 times by 3 different engineers in that time says much about the outboard business. The last bit of spline failed when we in Yarmouth so there was no way the flywheel could spin the crank
I really blame myself of course because the engine has never sounded 'smooth'. Also because it's a 2 stroke and I've never heard another the same to compare, I convinced myself it was OK. And at anything other than idle revs it was.
Thanks to Steve I now have a new flywheel fitted, some other mods and a full service done, so it should be business as normal. Can't wait to get back out
Can't recommend Steve enough. He's a mobile service and will travel reasonable distances from Southampton - furthest so far is The Gambia!
Also the money spent on Sea Start was worhwhile for me as a relative newbie to ribs and with limited contacts in the business.