Old Goat and others, thanks for the feedback. Internal kerosene heaters no longer in the running! Never considered the poison gases given off. When I was young living in Gloucestershire (many years ago) my parents had a stand-up round black kerosene heater going all through the winter inside the house! Years ago you used to see many such heaters around. But that was within a house rather than on a narrowboat, so much larger space!
I looked at electric heating but their operating costs get very high.
An alternative might be radiators with an Eberspracher unit feeding off my upper diesel holding tank (contents pumped up from the main underfloor tank). I looked at that in the past, but quite expensive to have someone install. I assume the air-pollution question is there addressed by having that unit's exhaust gases ventilate to the outside of the boat. Someone mentioned that there are suppliers selling used Eberspracher units and components needed with them, but I have not been able to locate any such supplier. Anyone have any experience in installing such system themself?
Anyway, thanks again for the previous feedback. Very helpful.
Phil