I had a prompt and civil initial response: an answerphone message from the head of RE assuring me that he would look into it.
Charlie has now had two RE lessons since and it has not appeared again. But apart from his teacher asking Charlie if he agreed with me (he said he did) we have heard no more about it.
I was expecting a reply to my letter (and one may yet come: I do understand that teachers are very busy) but I am pleased with the outcome anyway; no prevarication, no demonising of me or Charlie, no attempt to justify the use of an inappropriate film - simply putting the matter right and carrying on.
Of course, this is not a school with a Catholic Ethos...