Personally I have never heard of "Leisure Fuel". I always thought it was Agricultural Diesel.
The tax levied on vehicle diesel is road fuel tax. If you use an offroad vehicle for leisure puposes you can use Red Diesel provided that you don't drive the thing on the road.
The point that the RYA argued the case on was that it would be totally unfair to force the highest priced fuel in the EEC on Boaters. Don't forget that the major users of red diesel are small business men trying to eek out a living in difficult times for them. The concept of differentiating prices at the pump by some form of accounting system is so fraught with holes. Fiddles would be rife. We all know that the cost differential would justify all sorts of scams.
Of course the major pressure on Red diesel taxation must come fro the EEC who want to harmonise taxation & already have most countries in Europe heading down the route towards on price for all diesel. That should throw a few spanners in the works. The meerest mention of the EEC/Brussels seems to get the average Brit extremely hot under the collar.