Guys, thanks for all of the replies and suggestions.
Unfortunately I had a busy weekend so not much time dedicated to the Cobra. The replacement fuel pump gasket and
oil filter seal appears to have cured the
oil leak. I removed the PCV valve, cleaned with WD40, manually manipulated it and reinstalled. Removed the plugs and they looked OK, but difficult to really judge as I'd run the engine to move the car around the garage earlier in the week.
I went for a quick drive on Sunday evening (it's currently too hot here to drive it during the day!) and whilst traffic prevented a full test, the misfire appears to have gone. I won't declare this resolved yet, but it certainly seems to running better. Does it make sense that the PCV valve could have cause this?
Stay tuned and thanks for the contributions.
Kevin