The device works fine with the supplied software. My VI has worked with other serial devices and I have also tried the example serial/VISA VIs in LabVIEW (v6.0.2 and v8.2). I suspect that either the code to request a reading is wrong (the manual says use SPACE character) or the device may require an ID. The supplier and manufacturer have been little help, merely referring me back to the instruction manual. I have set up the COM port with the recommended RS-232 settings (9600 baud, 8 data bits, no parity, 1 stop bit) but get error 37 (device not found) in Serial VIs or Framing error in Visa VIs.
Problem solved! The manual seemed to be unclear. I read it as being set up for hardware handshaking but was, in fact, set to ignore this, with the DTR/DSR and RTS/CTS lines linked. Setting the RTS line to "Unasserted" has solved the problem.