Bid thanks to Lude-dude for the leads and quick postage. They arrived this morning (postman got me out of bed

) and I swapped them over for my crappy blueprint ones this afternoon. Also thought I'd swap out the blueprint spark plug tube seals that had started leaking oil down into the spark plug holes after just 8K miles. While the top was off I also did the valve clearances. The JUN cams are lasting really well, I can certainly vouch for their toughness.
The Magnecor's have completely sorted the misfire, and made the engine a fair bit smoother too. What's more I can now feel there's room for a bit more timing/tuning in a few parts of the ignition map. Nice one
