quite common this, for some reason garages dont seem to wonder why the cam cover gaskits fail. Age is an issue as they go hard, but the main reason, from personal experience and from what i have read on here is... Blocked crank case breathers!
These get full of mayo similar to what you may find normally on the filler cap with short journeys. This sets and blocks the breathers and the first place the pressure can escape is cam covers, then oil escapes and fills the plug wells, after a short period the ht leads rubbers cook in the oil and split, usually during the process of removing the oil. This leads to a missfire and an engine that wants to get out and walk.
First you need to clean your breathers or they will most likely blow again, make sure black gaskit goo is used sound the half moons only and not all the way round or the seal will be to good and the crank seal will go instead if the breathers block again, then once your sure the oil wells are dry change the plugs, vx are cheapest, and check the leads and rubber boots for splits or damage. You will probably need new leads. Motor factors for them or breakers on here possibly, vx want ton plus a side last i looked.
Try not to run the engine until any spark issues have been sorted or you may over load the dis pac, breakers on here for those if you need one.
The route cause of all of this seems to be vx lengthening service intervals to impress fleet managers and grab a few more sales. Change the oil regular once the work is done and your should have on more grief. Not sure what the oof recommended interval is but mine was done 3k ago when i got it and i will change it this weekend. I may then stick to 5k intervals(someone please advise if thats wrong)
This hole issue was the start of a load of agro with my old car, plugs, leads, breathers needed doing as said plus the alternater went as it was covered in oil,then the exhaust manifold started blowing but not sure thats related but it was directly under the worst leaking cover.
Hth
Ps. Imho you probably wont have any lasting issues with the oil in the cylinder, and its too late now anyway. I would be more worried about the breathers, cam seals and rough running.