Sorry, I dont know what "BIM2 tranciever.(www.radiometrix.co.uk)" is. In the case it is a radio data transceiver, think that the receiver is usually sending noise to the RxD pin in the 8051. So the SW in the 8051 must try to extract valid messages within the noise stream. This can be done using a header (for example 4 bytes long) followed by your data and terminated with a checksum or a CRC.
I have linked 8051s via UHF transceivers following this approach.
Can you give more details of your system?.