Sam, The measurement shown on your rule for the inner diameter appears to be about 27/32" which is very close to 55/64" which is the tapping size for 5/8 BSP in which case the TPI should be 14. I hope this helps but I guess 5/8 BSP plugs are probably few and far between.
I am pleased to say that I now have a suitable brass plug - thanks to Terry Holden. I still do not know what the thread is but clearly the thread on my inlet manifold is the same as on other MG J type inlet manifolds, and it works. Thanks again Terry.