I meant to finish this post with a photo of the head, wrapped in the bloody cloth, but I never managed to get a photo of it. I am pleased to say though that it was a success. Just before the head was brought onstage, the ASM dipped it into a bowl of watered down stage blood. It was gruesomely effective as the actor hoisted the head in the air and we could see the blood dripping down onto the stage.
To keep everything in good shape, the ASM wiped the head down nightly so none of the blood dried onto it, and the bloody cloth was put in with the laundry, to make sure it didn't get crusty.