As recently as yesterday I was coming up with nothing on the vehicle issues. The Maxton worked fine on a completely flat surface but any slight gradient would cause it to do all sorts of things that removed control from the player. This has been an issue for some time but now I think it's finally been resolved, thanks to a post on the 3Dbuzz forums from four years ago.
Basically, the 3Dbuzz thread found here suggests that the 'root' bone (centre of the vehicle skeleton) actually has an influence on the pivot and weight balance of the vehicle, and this was directly related to the problem of the vehicle 'rearing up' like a frightened horse. Moving the bone will influence how the vehicle behaves in the game engine.
I gave it a go - I moved the central root bone much further forward and re-imported the vehicle from Maya into the UDK, set the PhysicsAsset (which I was convinced was the cause of the problem before now) and tested the Maxton in-game on uneven terrain. Success! The car is responding much easier now, shows some signs of suspension in the wheels (though it is not visually accurate for the model) and doesn't come to a halt when it touches something as insignificant as a slight incline or a kerb.
This makes me very happy as designing the track and terrain - a vital part of the whole project - should now be able to proceed without having to compensate for poort vehicle handling. Fingers crossed that things will be a little smoother from this point!