Fault Code P0306 Cylinder 6 misfire

[SiWall]

Hello all, had the dreaded VSC light/check engine/traction off lights on my dash for ages. Manually cleared the codes myself by unplugging Battery which cleared the warning lights and car ran ok so long as I didn't put it under too much load. I was just putting off the inevitable so when I had the engine serviced this week I got the codes read, which came up with the above diagnosis. 

Mechanic has fitted a new coil pack on cyl 6 and cleared the code. However I didn't even get to the roundabout in sport mode before the fault came back. I/we are aware it could be injectors, wiring, plugs ( which I didn't do on the service - maybe someone can suggest some good but not horrendously expensive ones.....bearing in mind there are 6!). 

Is anyone aware of any of the most common issues with this code on this model (57 GS300) without the protracted investigation at silly ££££££££'s? 

Cheers