Has anybody had a problem with smoke from the exhust on start up when the engine is cold, it lasts for about 20 seconds

A common issue with all diesels, due to the colder seasons the cold engine will not burn the fuel efficiently enough hence the pale smoke leaving the exhaust, the 20 secs is enough to warm the mixture !

Worse case scenario would be worn valve-guides, but the smoke would smell of engine oil as opposed to raw diesel ;)

Sounds about right. The cold weather really does effect motors

Daz

agreed,nothing to worry about

Diesels will smoke more on start up if the glow plugs are down. A very common fault code on BMW but very rarely causes a starting issue as on other makes. A fault code scan would tell you this,

Yeah that does sound about right, I've noticed that I get it sometimes on my diesel when I attempt to overtake too

if it starts a little rough in mornings could be glow plugs. INPA will show up as 'Glow device'

But as said above, whiteish smoke is normal. Worn valve guides would be blue ish smoke and a distinct smells. Same for rotten turbo (plus you would smoke the whole street out)