"Because they are statistically trivial for _all_ UK boatingt activity combined, compared with just one week's output from aircraft"
Andrew,
I get really fed up with people making the comparison with aircraft as a way of justifying fuel useage on leisure boats. Yes of course aircraft use a lot more fuel and create more emmissions that mobo's - but does that really justify continuing to burn diesel at a high rate in leisure mobo's? It's that kind of "playground" thinking which results in a spiralling down situation in many walks of life. I still say that "two wrongs don't make a right" so any comparison with aircraft useage is, for me, infantile.
I hear what you say regarding your personal fuel useage meaning that the pricing will be an issue for you - but I would suggest that 40hrs cruising per week is a very high number compared to the average user and also using 200hp to go up the river to Windsor is mad - 10hp would do nicely. So I think your situation is a little extreme.