[syncro_user]

A thermostatically-controlled/vacuum-operated hot air feed (commonly found on old carburetted cars, built into the air filter housing) would also help. If I can get hold of one before winter it's something I'd like to try as well. Diesels suck a lot of cold air otherwise, which slows the warm-up cycle right down.

edit: actually thinking about it, without the heater your engine will be using diesel fuel from the tank to warm itself up anyway, while the engine is working less efficiently, thus giving worse fuel economy. Run the heater, decrease the warm-up time and you're ahead. It probably is controlled by the ECU, but standalone controllers may be available.