Do you not think that in general there are better ways of getting cars to slow down other than just speed humps? Maybe they would cost more, but what about chicanes? However slowly you go over some humps (e.g. bottom of Thorpewood) you can hear your suspension groaning. Agree that speed cameras might help but it appears that many of them are not working (e.g. Brenchley Gardens) and drivers get to know this and ignore them.
I have great sympathy for those around Derby Hill and it sounds as if residents parking might be a good idea as well, but that can also be a nuisance when you have visitors/workmen and have to sort out visitors passes.