DO NOT RUN AN EXHAUSE INTO THE COCKPIT - it could be the last thing you do as you could fill the cockpit well with CO which will then drain down the companion way where you are sleeping...

That is a fairly serious heat requirement, probably about 5Kw if you want to be snug in a Scotish winter but does depend on the level of insulation. You are also ideally going to want something that can both push out enough to warm the boat quickly and then be able to shut down and tickover 24/7 to keep it warm and dry. Nice thing about the eber's is you can fit a thermostat but you would be looking at the bigger sizes. The speck they give about boat size is based on spring/summer use for winter you need to about double it accrding to the canal people who seem to be the experts on boat heating. The nicest (and cheapest) option is a log burner and if run on charchoal is quite clean but as you say finding room for one is difficult. There are bulkhead mounted charchoal burners which are more compact but only about 1.5/2Kw. Gas is hopeless unless it has a built in flue as it liberates 1L water for each L gas burned so you get a sauna. The taylors diesel/parafin bulkhead stoves are probably the most compact acbin stove & might be a possibility? very reliable and simple with good heat output and small size flue. If you cant mount then near sole level you can always fitt a small fan to blow the hot air down.