Hi my fabia 1.9 sdi has just started to misfire plus white smoke from exhaust (badly) there is no oil in coolant bottle,no mayo in filler cap/or dip stick.Could this be head gasket still or a injection problem,wondering if anybody has any ideas where to start??

What tells you it's misfiring? I also have an SDI which produces a lot of smoke and wobbles a lot for the first couple of minutes but then always settles down.

Actually, how can a compression engine misfire? I thought the combustion happens when the piston reaches the top of the cylinder, so I'm not sure how it can fire at the wrong part of the cycle. Or is it just not firing on all cylinders? Injector / pump issue, maybe?

No doubt someone who knows what they're on about will be along soon.