When you say left hand side, is that looking at the engine? If so you mean the plug by the timing belt end?
Which is by the oil filler?
Just a guess, but last year I had my car serviced at a well know tyre fitting company and they managed to spill most of it over the engine, then forgot to do the filler cap up so I had a plug well full of oil.
I took it back within minutes, but it resulted in spluttering, chugging and a week later, a fooked cat.
If you have done an oil change recently, it could be that.