After much testing I have come to the following results which I hope make some sense to you.
It seems that the PLC is interpreting datagrams meant for addresses: 1,2,4,5,7,8,11,13,14 (DECIMAL) as something other than MODBUS. When any of these addresses are transmitted to, all PLC's try and respond. Address numbers higher than this seem to be unaffected.
If I force a PLC to only use MODBUS via the setprotocol command the PLC does not respond to any address request apart from its own. This may sound like a solution but I would like to be able to program the PLC's via radio and hence don't want to force MODBUS.
The above also holds when I try and communicate with a TriPLC rather than the SCADA package.
Do you have any idea why these address values seem to be problematic?