They confirmed it is a factory fault from 2011. It seems the drive shaft bearing has been pressed onto the drive shaft incorrectly. Had the dealership who sold me the outboard picked it up, as they should have, the engine would have been replaced immediately under the warranty. The warranty lasted four years so up to 2015 - so my regular service engineer chandlers should hang their heads in shame - they have taken my money under false pretences!
This diagram which the national team supplied me shows the offending bearing.
They would like to see the unit for themselves so that they can assess how they can put things right and prevent it from happening ever again. I told them I had done my own service this year - engine oil, spark plug, greasing moving parts and draining gear lube. They seemed happy enough and un-alarmed about this.
I suspect that they will either send out a kit of gear components, free of charge, for me to fit; or they will ask me to send them the lower unit and they will replace that completely with a new one for me to just slot back into the upper unit. I hope it will be the latter solution.
I have to say I have been very impressed with the speed and care of service from Tohatsu UK and Tohatsu Japan. Full marks so far. Amazing care and attention to detail. I'm hoping we can resolve the situation in the next few days.
Despite popular belief, I rarely use my outboard - probably about 12 - 15 hrs max per year. Mainly used for leaving the Sutton harbour area and out into Jennycliffe Bay and the return journey at the end of the day. The longest continuous journey it has made is from Cargreen on the river Tamar back to the Cattedown; and from the river Yealm back to the marina at Sutton harbour (around 7 - 10 miles max each time at less than half power). Yes it is scratched around the shaft and the cowling. It looks a little worse for wear in places but on the whole I have looked after it well. It has been serviced for six years at least, never been dunked and nearly always washed down and flushed out after every trip. I nearly always drain the fuel and carb over winter as well. The only thing I needed to replace in all this time was the plastic integral fuel tank which developed several hairline cracks around the screw neck cap area.
All that remains of the service I am doing this month is to take out and clean the carb and check the fuel filters. Maybe I'll wait awhile until Tohatsu have sorted out what they want to do.