owain. It's remarkable these gaskets seal at all, considering the narrow width of the contact areas. First check is that rear tappet cover and proper identification of the source. A good clean, a short run, a looky look.
Have a good look at where the leak is worse, see if a feeler gauge can be worked in to identify any area where the clamping force is less, on both sides of the gasket.
Worth checking the two rear nuts are doing their job, and not bottoming on the plain central part of the studs, rather than clamping, or worse, that the internal threads in the rockerbox are about to let go. The single bolt inside the rear cover also needs a check....is it clamping or bottoming on some old gasket sealer?
If you take off the rockerbox, see if there are areas of gasket which are less compressed, indicating a low spot. Removing the studs, cleaning the mating face and seeing if it rocks on a sheet of glass is worth a go, feeler under the face for low spots.
Replace the rockerbox without studs, gaskets or pushrods and feeler gauge between the two, looking for low spots and rock on the head.
Providing the mating surfaces are true, a thinner gasket as suggested by Beezermac is perhaps a better choice to achieve a higher and more consistent clamping force over time.
If it comes down to a reface, plenty of how to do it tips. Raised areas around the studs need to be flatted back. Sticking the gaskets to the rockerbox with silicone, and leaving to harden on a flat surface before trimming away the excess is also another trick, but has an obvious risk.
From you description this is a definite leak, rather than a "weep", so something that's definitely amiss should not be too hard to find.
Swarfy.
