Bollocks. I reckon it's Helicoil time. It wasn't me, I reckon some PO tried to "cure" the old weeping valve cover gasket by "nipping them up a tad". There was bugger all left, and the new gasket is leaking pretty badly near there. Anyway, no point crying about it. I don't want to do this myself, so I will drive the car to my trusted indy, assuming the gasket leak isn't too bad when I finish putting it back together. Or ask him to do the work at mine, then at least I can save some labour cost by taking off all the bits that need to come off to get access.
Can I confirm that the valve cover must come back off, so that the head can be drilled for the helicoil? I'd love it if it could be done in situ, but don't see how that would work.