most of the vauxhals had octane plugs, used to put the highest spec one in you could find. Must be an efi xr3i on that age, some did have octane plugs. Not sure this would cause a missfire though, even if it was unpluged. I would be looking at leads and correctly gapped new plugs, as everything else on that particular edis ignition system is pretty bulletproof (assuming it is efi, with edis4 wasted spark) be interesting to see a pic of what he think is to blame though, nothing of this nature would put me off buying the car though.