Recently there was long thread about the BCA. I can now post possible answers to some of the questions, such as "Why is the BCA still raising membership prices when there is a significant amount still left in its coffers?"
I *think* it was M16, so 24mm head? Sorry, I should know, I did organise the replacement of it a few years ago but can't find any notes. A search of the forum here might bring up the exact size as it was discussed at the time.