I've recently put a new rocker cover on my car, and I was having trouble as I tried to use the old gasket. It obviously didn't create a good seal, and caught fire. Luckily we saw this and turned it off before it could properly catch alight.

So today my new gasket came and I put it in and ran it for a while and it was all ok. So I turned it off and did another job and turned it back on and it started smoking loads straight away. Why?

Could be oil on the exhaust manifold that's burning.

Is there any way to tell for sure?

Do you think I should take the rocker cover back off and make sure both the surfaces are clean?

I had the same problem on my car. I changed my rocker cover on my engine so that i could paint my old one.
When you remove the rocker cover it allows engine oil to run onto the exhaust due to the engine being angled towards the exhaust.
The best way i found to do it is to allow the engine to cool down completely and place some clean absorbant rags over the exhaust manifold to catch the excess oil. It will still allow alittle bit of oil onto the exhaust but not alot.
When you start it from cold remember to remove the rags first and leave the bonnet up and allow the engine to tick over and it will burn all excess oil off until it stops smoking but always keep a close eye on it just incase.
I hope this helps.

I did exactly as John said when fitting my new gasket. Like he said, you won't catch quite all of it, by the rags definately help

(Have to admit, it is very unnerving watching the smoke billow out from the back of the engine when firing it up after changing it )

Me too, only don't do what i and some friends did, and put the cover back on, seal it all back up, bolt it down, then take the rag out to discover you've caught some of it between the 2 surfaces! :blush: