After a few seconds it goes back to the menu screen. I've reset it and recovered the OS but it's still doing the same. I suspect it's connected to a large image file sent to me but I'm unable to get into the messages to clear this.
It seems to have started working now. I hit the pop up for another message as it came in, this opened and from there I managed to get back to the main messages page. All seems to be working now, but not sure why!