OK so… I now have a solution for this hole (plug it and fit a temp sender in the radiator hose) but I’ve found a new hole, in the inlet manifold.
As you can see from the picture, there’s a hole, and there’s a similar one the other side which has a little grub screw in it. There does appear to be a thread on the hole, but I’m wondering whether it should have something in it? There’s also a hole the other side.
Thanks for the response, that’s what I wondered but there’s a similar hole the other side, actually two. Seems a bit odd to have so many? There’s only one grub screw (the one you can see in the picture), all the others are empty.