@Sean d MPG has remained consistent at around 35mpg and there's been no other noticeable issues such as power loss / noises that would suggest that anything was wrong. Rough idle on a cold start is common amongst the 1,9 zed variety so I didn't think much of it.
I'll have to check the other plugs tomorrow.
@t-tony is the plug lead the same as an ignition coil?
Euro have them, but they're a set of 4 that is around £90 and I have a feeling that I'll only need one.

I've put an order for a new plug -- as the one I put back in landed on the under-tray and I'm not a huge fan of that being used again -- but what I'll do is I'll swap out the ignition coil as well as the plugs before I start the car tomorrow. Fingers crossed that fixes both issues. It's good that I have two identical zeds, incredibly easy to diagnose issues in a way.
