Outer Limits 4x4 Board

Hi I'm in the process of removing the head of the 3.9 litre V8 on the Disco.
WHAT IS THE SOCKET SIZE for the head bolts?
16mm seems a little loose, 5/8 incn seems close?
The last bolt near firewall end is vey tight. I don't want to strip it.
Maybe my multi sided sockets are the wrong type.

16mm , but had to have flat inners and not multi pointed.
I used one from an impact wrench it fit perfectly.