It's funny how convention regarding the bell differs from area to area. Certainly on the old Little White Buses around West Lancs, ringing the bell was considered rude - you got up and asked the driver to stop. Yet on almost identical MK Metro hail and ride routes (now mostly replaced with fixed stops), it was de-rigeur to use it to mean "stop at the first point you consider safe" and considered odd to distract the driver by talking to him.
I also got told off for using it on a Blackpool tram once - you were meant to ask the conductor who would ring it.
I once got a real b*****ing from a Preston Corporation conductor for ringing the bell. Must be a Lancashire thing!