I do not believet that there is any way to directly get a Unitronics PLC to talk BACnet. Just as a quick education for those who may not have ever heard of BACnet. BACnet is a building communication protocol that was developed as a "desired" standard for the United States market by a group comittee process from ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers). The standard has not been wholey adopted and has been in fairly heated competition with the other communication protcols (LON works, Johnston N2-Metasys, proprietary Siemens protocols, and modbus). Whether or not BACnet will ever achive the desired conformity is unknown but unlikely. LON has been much more successful to date at getting their standard adopted with a much wider variety of gateway products available from multiple manufacturers.
At this time the easiest solution is to buy a gateway product such as a Fieldserver product such as the "Protocessor" and either go through the painful process of mapping everything yourself, or paying Fieldserver to do the maping for you. In general if you have Fieldserver do the work for you it will run in the $5000 range for a single project for the hardware and sofware. There are other solutions available but it is not an easy slam dunk solution that I would be able to say definately do it this way or that way.
Sorry
Keith