Not necessarily - while if you have line of sight it obviously should work, as long as you're relatively close and the airfield has a powerful enough transmitter you may still pick it up anyway. For example at just over 2nm as the crow flies, with no direct line of sight (buildings + trees etc) I can pick up transmissions from the tower, though not from aircraft on the ground.
The phrase "line of sight" when applied to VHF communications, doesn't mean that you have to be able to visually see the transmitter, but that you are inside the radio horizon.
Obviously, it will be possible to receive ATC further away than you can hear aircraft on the ground. The tower building at Cambridge is 8 stories high I believe, so as the effective "line of sight" range is a function of the height of both the transmitter and the receiver, that is bound to happen.
For an idea of range, try putting the height of the tower and your receiver into this website.
VHF/UHF Line of Sight Distance Calculator