Why thirteen bells?

Someone asked why St Mary's Hitchin has thirteen bells in the tower. I wrote an answer, and I'm putting it here in case you're interested.

Background, which you may know: you can begin a scale with any note on the piano, but in order to sound right you'll have to use different combinations of black (sharp/flat) and white notes for each starting note.

The original ring of eight at Hitchin was in the key of E, which goes

E, D#, C#, B, A, G#, F#, E

In 2012 they added four more bells, which were lighter than the original set, and so had higher notes. So now the sequence goes

B, A, G#, F#, E, D#, C#, B, A, G#, F#, E

Obviously you can ring the back eight as if the new bells had never been installed and it'll work fine. But if you want to start with the new treble, you have a problem, because you're starting at B and so the pattern of black/white notes is different. The key of B goes

B, A#, G#, F#, E, D#, C#, B

And you see that all those notes are already present, except A#. So they also put in a bell tuned to A#, which lets them ring the front bells tunefully.