off with her head!...

 wet fuel deposits

 sooty exhaust valve

 moisture/fuel? droplets on inlet pushrod

 groove in rocker cover

wear on edge of the exhaust valve collet

Finally decided to take off the rear cylinder head to try and locate anything could be causing the misfire.
After speaking to my technical advisor (heh heh) for pointers to look out for, I mentioned this mark that had appeared again on the cover wall.
If you check back on my build last March (head job) I relieved this area of the rocker cover after it took nearly an hour to get the cover off because the valve collet was jammed against the cover wall.
Is this the cause of my misfire? Is it holding the valve open? Is the valve the correct length?
My valve spring compressor that worked in the past slips over the valve stem keepers on these alloy collets so no removal today.
I'll keep you posted on any further exciting developments...