VW LT46 diesel minibus - won't start below freezing?

Turns over in the morning but wont start if it drops below freezing at night. No warning indicators.
Put heater under engine pointing up (on opposite side to exhaust outlets) leave for an hour and it starts first time and is ok all day long. No problems during summer.
Tried diesel cold start additive etc no joy.
Tried heating up different areas with hair dryer, no joy.
Tried 3 different garages, including VW, no joy.
Someone suggested a relay (part of immobilizer circuit ?) gets stuck when cold.
Any ideas ?