Ive never had to go into this with Derbyshire County Council, however now that I own a Z and the speed bumps may cause me issues, I may need to.
As the law stands, Speed Bumps are in place as speed restriction measures, i.e to ensure traffic is restricted to the legal limit. It is the law that all speed restriction measures should be constructed in a manner which allows them to be driven over at the posted legal limit, without damage to the vehicle or injury to the occupants.
If you cannot drive your vehicle over a hump in a 30 MPH zone, without causing damage to your vehicle, the speed restriction measure is illegal, and should be removed, if it is not removed, and by driving over this hump at the legal limit or below, you damage your vehicle, the local roads authority is liable for the damage.
If you write to your MP on this matter, he or his office are obliged to respond. I'll see if I can dig out the actual legislation...
It may be worth writing and asking if a member of the highways authority is willing to accompany you on a tour of your local town, driving over any speed restrictions you may encounter, at the legal limit, and at the end of the tour, would they be willing to personally pay for any damage caused to your vehicle. If they decline this offer, then it can be proven, to a degree, that they do not have confidence in their speed restriction measures being fully legal - if this is the case, you can then take it further.