406 diesel won't start in cold weather

[momin]

my 406 diesel turbo(N reg) won't start in cold weather last winter. changed glow plugs in march. worked ok throughout summer. since last week, because of cold weather, it won't start. I have to spray cold start and it starts. works ok all day. the relay is ok and does cut off after about 7 seconds. i've being told cold start spray is not good for the engine. please help, what can be wrong that it doesn't start in the morning in cold weather only.