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Our nissan primera doesn't start at times especially if the petrol is in reserve but not empty. We have replaced the spark plugs answer that helped but it has stared playing up again. We have to crank the engine for about 20 seconds with the accelerator flat down. When it does start all the smoke comes out.

There are many possibilities, so without carrying out a diagnosis it's hard to say for sure. Sometimes, flooding can be an issue which will cause the engine to crank over for a long time and then blow clouds of smoke when it finally starts. This can come about if the previous time the car was started, it was only for a few seconds to move it - perhaps to let someone else's car out or moved briefly out of a garage to wash the car? As a cold engine needs a rich mixture, this senario results in 2 lots of rich mixture injected into the engine, causing it to over-fuel.
If this doesen't apply to you, it could be something more serious like low compression, stretched timing chain etc, or perhaps a fuel pressure/delivery problem - but as I said, without diagnosing the problem properly, it's impossible to say, so may be a job for your mechanic.
Howe