Smoke after gasket change

[London 5]

Hi, I've just replaced my alternator and rocker cover gaskets.

On start up I have thick smoke coming from the rear left exhaust, allot of it.

I did the gasket change because of oil in the spark plug, the rear left plug did have allot of oil.

do you think it's the oil burning off or something else?

I've checked the gasket and them seem to have sealed well.

Just had a look at the spark plug on the smoking side and i do have oil residue on them. I checked the first three. Car was working fine before (part from the alternator ofcourse). I only changed the rocker cover gaskets as I had to wait for the alternator.

Thanks