Well we seem to have come to the conclusion that most Defenders have inefficient heaters.
I would love a Eberspascher or Webasto, but can't justify the expense! As my 110 is parked outside my garage which has mains power, either an oil filled radiator or a pre heater device fitted to the engine will have to suffice on cold days! I will look into the options.
Insulating the rear of my 110 TD5 hardtop made a huge difference, however I would say that the truck came with heated seats as standard and these are brilliant ! You do not need heated seats once the engine is up to temp!! To be honest the heated seat can get too hot!
Radiator blinds; I've not tried one, but fitting a rectangular rubber sheet over part of the rad grille (avoiding the inter cooler) worked really well on my previous 300TDI.
What would be great is shutters/blinds over the radiator that open/close according to the air temp. Cars in the 50s I think had rad blinds operated via a chain on the dash.