[05ZETEC]

I have a 2005 ford focus mk2 (LS), an recently the engine has been having issues. When I first bought the car it couldn't start on it's own and needed to be jumped every time it was stalled or turned off. Now the alternator has charged the starter and it can start on it's own steam (Admittedly not very well) but when Idling the car has a very noticeable misfire (It sounds cammed and the entire car rocks side to side). I have had a warning pop up on the info system warning me of 'engine fault'. I have charged the battery and changed the spark plugs (The car has a spark plug well leak which may affect the spark), these have fixed the misfire in the past but it just keeps coming back. Should I buy a 'new' second hand battery for it or should i fix the leak in the sparkwells first?