Head bolt tighten - long socket?

I need to check/tighten those two front head bolts that are tucked down in the fins between the exhaust ports. I can tighten one with a long metric socket because the bolt is new replacement but the other one I assume is original witworth . Does anyone know exactly what size would fit in either metric or imperial? I only have a few long reach sockets and none of them fit.
Thank you all.

Hi what you need is a 1/4" whitworth tube spanner, this will let you tighten them with the tank on.
Peter

Thank you Peter, I found one on ebay for less than $10 including shipping! - done

You don't need a deep socket, simply a standard 3/8 drive and a shortish extension. A 1/2" drive socket is probably too fat to fit.