You were very lucky Keith. Had something similar climbing out of Bournemouth a few days before Christmas nearly 30 years ago en-route to Brussels and in IMC. Engine started missing badly, called PAN PAN and was vectored back to BMH ILS. After safe landing with fire engines behind, taxied over to my maintenance guys who found two faulty plugs on one cylinder! Turns out the top one had lost electrode which had dropped onto plug below. Was told I was lucky that my Piper Arrow had a six cylinder Continental as with a four in same situation would have been lucky to maintain height.
