Some may scoff and turn their noses up at such things, but being heard on a bike which can often be silent is very important, not least for pedestrians but also for motorists. A good bell can make the difference between hitting someone or being hit, and other road uses being aware of your presence so that they can take appropriate action.
As usual I did plenty of reading on the subject online, it took me a while but I now believe I have what I need so far as bells go.
On the mountain bike (yes, I use a bell on my mountain bike) I have a Cateye PB-1000AL, it is a very loud and efficient bell which gives two dings for one pull on the lever, which is also very easy to operate; it may sound silly but some bells are hard work to get a loud ding out of. This bell is mainly there for the courtesy of pedestrians who also use the forest where I ride, they will normally shuffle out of the way when they hear it; the same goes for slower riders on the trail ahead of me, although it feels a bit mean ringing my bell at them (feels like I am commanding them to move over!) I think most prefer a polite bell ring rather than hearing me breathing down their neck in silent judgement, or turning around to see me right behind them , giving them a fright and causing them to almost fall off into a ditch as they hurriedly try to get out of the way (it has happened in the past!)
For the road bike things were a little more difficult owing to the thick 32mm bars, most bells simply do not fit this size bar, and the space available is severely limited where the bar tape ends and the stem head begins.
After buying a few cheap bells and looking at possible ways they could be modified to fit, I came across an offering from Rock Bros which has a wide adjustable fit system and is also a very loud and simple bell to use, perfect! I used a piece of old inner tube to protect the bar from the metal strap. The actual strap footprint is very small so should fit any handlebar.