1. A trunking radio system comprising a central station and at least one remote unit for communication with the central station over a plural number of frequency-divided radio channels, wherein the central station comprises: a transmitter for transmitting a plurality of signals to different remote units on different channels; a trunking controller coupled to the transmitter for allocating channels to said different remote units and for signalling to a remote unit via the transmitter to access a channel, said trunking controller having traffic monitoring means for monitoring at a given time the number of channels allocated, said trunking controller being responsive to the traffic monitoring means to signal to the remote unit via the transmitter to access at least two channels when the number of channels allocated is less than the plural number of radio channels; and diversity means responsive to the traffic monitoring means, for receiving signals from the remote unit over said at least two channels; and the remote unit comprises: a receiver for receiving signalling from the central station, and diversity transmitting means responsive to said signalling for transmitting a signal over said at least two channels.