Jimmy Jammy, Spark Plug bud, get one. You most likely oil fouled the plug (because of the higher oil mix for first two tanks), and after cleaning it you MAY still get a spark so you think it will work. Wrongo, the spark will not be strong enough to do the job. Trust me, I have been right down the same path your at right now. Get a plug and let me know what happens. I am not sure about the UK but I found that our size plugs are pretty special. Local auto stores and HS didn't have it in stock. But I found my local auto place could have it the next day, and for less than a HS. I ordered two, since this is my case. If you find it's rare, make sure you order a couple, no one likes down time!!

Paul