Thanks for your reply. Yes, it's the lovely dark blue 4v that's been on car & classic and Ebay for months.
Luckily he left the old rotor, cap and leads which to be honest apart from one odd lead appear to be better quality than the new ones, which were over £600 from Eurospares. The new leads don't "click" into the cap or coils, not great at that money.
Interesting about the spark jumping to another cylinder, I haven't heard of that before. I don't think the distributor was disturbed when he fitted the new kit but it's worth a look. It is getting a spark down the No6 lead during the misfire in this case though, I checked with a timing light clamp on the lead.
I'm still hoping it's a faulty injector, or it could get very expensive!