in order of likely hood id say
most likely
plug
coil pack
less likely
injector wiring
injector
HT lead
even less likely (but still possible)
Piston rings - smoke
Headgasket - smoke/mayo
first find the affected cylinder, unplug ht leads until its clear which cylinder is misfiring (DO NOT PLUG OR UNPLUG HT'S WITH THE ENGINE RUNNING!!)
unplugging a HT from a healthy cylinder will cause the misfire to get worse as you drop to 2 cylinders!!
unplugging the unhealthy cylider will cause it to stay the same. the change might not be obvious as the engines can run on as little as 1 cylinder i found, so might even end up just trying each cylinder plugged in on their own.
after youve found the cylinder thats missing, remove the plug, rest it on the engine while still plugged in to the pack, and crank it. If theres no spark then youve found the issue. try swapping plugs and leads around to find out if the problem is the plug or HT, if neither it will be the Coilpack, theyre a pretty universal item and quite fond of going wrong.
if its a spark issue, then itll cost under £50 and easily done yourself.
If its not the spark thats the issue, youll have to look into fueling! first try simply removing the plug to the injector on the affected cylinder and inspect for damage or bad connection, (i had a miss fire due to a cracked plug on the injector once). once again if this is not the case then you will have to find another injector to try to see if it isolates the issue.
hope this helps!