1. Apparatus for transmitting data blocks on a communications channel having a radio link between the apparatus and at least one user equipment, comprising: a receiver configured to receive first data blocks from the user equipment; a transmitter configured to transmit second data blocks to the user equipment and to transmit polling messages to the user equipment to request that the user equipment acknowledge receipt of the second data blocks; a transmission controller configured to dynamically set at least one polling interval for transmission of the polling messages to the user equipment after transmission of the second data blocks, wherein the transmission controller is configured to set the at least one polling interval responsive to at least an estimated used transmission capacity value for a radio coverage area of a cellular radio network in which the apparatus is located.