Hi all. I had no end of trouble getting this stupid thing to work, but it turned out to be really simple.
It uses an LP2951 variable regulator to supply the PIC micro from the various COM1 (serial) ports. Having measured the voltage with the PIC removed (4.1V) which was less than Vmin of 4.5V for the 'C54 series, I tried lowering the 24K SMD resistor between pin 7 and GND by soldering in parallel a 100K resistor. This brought Vcc up to around 4.75V which was then enough to get Windows to detect it and install the drivers.
If anyone else has one of these which is intermittent and/or won't work reliably then this is the fix. There seems to be no reason why they chose 4.1V as there's over 7V available
Regards,
-A