Well, thanks to all who've commented so far. Consensus seems to be that any conversion is going to cost a fair amount and cause me some headaches. I'll probably never recoup the investment.
Engine was serviced (major) 18 months ago by Mercury service agent in Glasgow. Only run about 20 hours since then. Oils changed, plugs checked, filters checked and replaced, greased, etc. this season. No fuel flow meter so have to rely on dip stick after each trip to measure consumption. Figures so far are :-
3500 rpm 23 knts 12 lph 7.2 mpg
3100 rpm 20 knts 11.3 lph 7.1 mpg
Only 8 hours running time, so far, so figures could change. All measures in light airs and no swell to speak of. Just me on board and 90 litres of fuel to start with. How do the figures compare?
The pipedream had been to buy a cylinder of propane and decant contents into a fitted tank on the RIB.
The manual uses half a page warning of dire consequences (for the engine) of using alcohol in the petrol. I think it's possible to make bio diesel in the back garden from old cooking oil. Not sure about bio petrol. If possible, it's bound to be illegal.
Hasn't there been a thread somewhere about adding parrafin or kerosene to the petrol? I wonder what that does to the engine?