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.
You're going to have to weld the vertices back in to place. There aren't that many, but if you're trying preserve your existing UVs, I don't think there's another way. I would turn on the Sync UV and Mesh Selection button and select a vertex. The vertex that it should be welded to will also highlight. Turn off the Sync button, move the vertex that is out of place over to the one that is in place and weld them together. That will work for the verts on the edges of the rips, and then it looks like you will have one or two loops in between the ripped edges that you will have to use your best guess at placing.
Edit: It looks like several vertices are mapped to a single point. You'll need to spread those out so you can see each vertex individually. You could possibly create new UVs for those to make your job a little easier. Turn on slection Sync, select the bad faces in the UV editor and unwrap in the 3d View. This will give you something cleaner to start with.
What you are talking about is that you selected a face or mesh part and it came of with extra vertices, so accordingly when you select one of them the others get selected too, right?
This can happen when you are in face select mode and UV sync selection is on. To fix this you have to unwrap again with seams marked but this time turn on Live Unwrap, unselect the UV sync selection, select the ones you don't want rearranged in the editor and press P for pinning. This way it will keep those pinned vertices the way they are when unwrapping.