I have been caught speeding by a mobile unit parked at the bottom of a steep hill just in front of a speed limit sign. It seems to me that the Police are trying to ambush motorists at this location. Can I defend/contest the allegation on that basis?

Whilst you may not like the location chosen by the Police, that does not mean that there is a viable defence. The only issue the Court will be interested in is whether you exceeded the speed limit. The Police will justify the position of the van on the basis that offences are committed at that location. The Court will expect drivers to take into account any gradient when monitoring their speed so if the speed alleged is accurate, the case will be proved.