My car won't start in a very frosty mornings

You have verified that the glow plugs work?? I really doubt it.

Possibly a weak battery or poor earth / starter wiring that's only showing issue when current demand is high.

Probably not enough CCA cold cranking amps from the battery ? Check the manufacturers proper size battery for the vehicle. ...

only takes one glow plug out of order to cause problems . check them for proper volatge and resistance

sounds like a bad sensor. the glow plugs should stay on long enough to reach a certain temperature, especially when it's cold. So turning off after 1 second is not nearly enough. There may be a problem with sensing the temperature of the glow plugs.

does the glow plug/coil light flash at all???? once would indicate glow plug relay fault! three times would indicate FUEL pump failure! sorry never experienced two flashes....

Kind regards

IF YOU ARE HAVING PROBLEMS STARTING CAR LIKE IN MORNING.YOU HAVING GLOW PLUG PROBLEMS.CHECK GLOW PLUGS RELAY FUSE. .REPLACE AIR FILTER AND FUEL FILTER.IF YOU STILL HAVE PROBLEMS.YOU HAVE PROBLEM IN FUEL DELIVERY SYSTEM .LOOK FOR FUEL LEAKS AT INJECTION PUMP.YOU CAN HAVE PROBLEMS IN SHUT OFF VALVE.MAKE SURE NONE OF FUEL LINES TO INJECTION PUMP DONT HAVE AIR LEAKS.

Since you're running diesel, you might need an engine block heater. Even the largest vehicles need to keep the block warm when the winter temperatures drop. It keeps the engine oil & fuel looser.