Found the thread gauge as Cookson had leant me his tap n dye set. Mine only went up to M8. Thankfully it had a imperial dye of 1/8 27 NPT what ever that is but it fitted. If I'm being honest the fit worked brilliantly on the new head that I had bought but the fit is tight on the plug so I've used the dye and needed persuading but it does fit..
So I've going to get this kit so hopefully should be able to tap a new hole and fit a helicoil. Fingers crossed.