In reply to phja:
you might already do this but one trick which can help is to turn key to arm ignition (and the plugs) but stop one click short of turning the engine over. Wait for plug/coil light on dash to go out (will take longer in the cold) then switch back bff, do the same again but turn all the way.
> Am I right in thinking that by only having to heat the glow plugs rather than turn the starter motor there'll be less demand on cold battery and so more likely to start?
pretty much - when you turn the engine over, the starter motor takes a lot of current which causes a volt drop across the battery (internal resistance). This in turn reduces the glow on the plugs. Same reason headlights dim when you start the car.