>> MAME complains about an invalid header which means the save state was made with a different version of MAME or a version of 0.150 which has been adjusted in some way
It would appear something has changed between 0.150 and my build of the latest revision [SVN r25399].
The state works with that version and causes the problem.
I've attached the .exe as a .zip but I can understand if SVN versions are off limits.