P0302 misfire in cylinder 2 fault code

I changed the spark plugs as part of a service on my 2003 mini one, ever since then it started misfiring and stuttering etc.. I put the old plugs back in to check the plugs weren’t at fault and it’s still doing the same, I just run a compression test and as I pulled the plugs out plug number 3 appeared to be fouled. The test results showed 155 psi in cylinder 1, 165 psi in cylinders 2 and 3 and 180 psi in cylinder 4 which is ringing alarm bells for me, could this be what is causing the misfire or could that be a different problem or is something else causing the misfire? Any help would be much appreciated. Tia