Thanks for your help. Problem solved !!
I followed your advice and compiled an EXE to test the system. When I ran it on an LVRT PC I found it was looking for several other dll's. Upon investigation I realised the required dll's are all part of NI-DAQ (mx). Once I installed the NI-DAQ drivers, everything worked, including the original dll.
I assumed the DAQmx VIs that were included in the build list (for the dll/exe) eliminated the need to install NI-DAQ on the LVRT PC. My mistake.
Thanks heaps for your advice and help.