I seem to be having trouble with one of my UV meshes. For some reason, a number of vertices/faces have decided to detach themselves and, not only are there too many, but I can't re-attach them. The simplest thing to do is to unwrap the mesh again, but now I can't get it to align properly. The save I've attached is with the broken vertices... Any ideas what I could do? Thanks.
I don't think you can avoid re unwrapping.
The reason why you have the separate UV island in the head unwrap is because that is the way the UV seams are marked at the top of the head.
To make the head a single UV island clear the unwanted seams and mark a new one along the top of the head as in the screenshot below. Then UV unwrap again.
You will have to experiment a little to have the exact seams to get the correct unwrap island shape and then scale to match (align to) what you had before.
Just a note : usually the ears of a head will need UV seams so that they will be separate islands when unwrapped.