We have hot air central heating - new last year, very efficient if not a bit noisy. BUT it doesn't really warm the bedrooms. The air duct only extends to the hall upstairs, the idea being that we leave the bedroom doors open to let the air go in - but it is not enough. What could we efficiently supplement the heating with?

I would have the duct checked first. It could be dampered down or a register in the floor could be covered up. A small electric heater could cure you no heat problems in the bedrooms if nothing is found with the central heat system.

You say you have no supply ducts going to the bedrooms, you may want to see if you can add a duct to the bedrooms, adding electric space heaters will significantly raise your electric bill, this would be a last resort fix.
If you have an attic space above your upstairs you might be able to install some duct work from the ceiling of the hall where its warm through a duct booster fan and into the bedrooms at the ceiling, good luck ,Mike