A quick review for those who might be interested and following this topic:
Mercury tiller extension recommendations?
I bought one of the eBay recommended ones at £20 posted:
Telescopic Outboard Motor/Tiller Extension Handle 17"-25" | eBay
And it's not too bad. Reasonably well made and fits the large diameter Mercury/Mariner handle like a glove. The extension comes with a rubber insert for thinner tillers too but this was not needed which helps it feel more solid, the thumbscrew works well and stood up to some rough conditions at the weekend.
However, two concerns; firstly the socket is secured to the tube with two rivets, as these are opposite each other they allow a bit of slop in the fitting, I added two more rivets between making it much more secure and solid.
Second is more potentially dodgy - it 'locks' using the usual twist either way method with an eccentric plug inside the tube, same as the telescopic walking sticks. It is possible then that if not *really* tight it could possibly loosen in use and undo at a critical moment of throttle control. As the tiller extension was the perfect length at the shortest point, I locked it tight and simply drilled a 5mm hole to insert a SS bolt right through locking both parts together. It would be easy to make another hole or more further along if you did want a choice of lengths.
Anyway, in use it works great - I would say it's essential for a SIB as all the ones I have had needed some sort of extension to get the driver forward more for comfort and performance.