In regards my problem with the SIO2PC-USB which I documented in tedious detail above; they still occur but I have largely obviated their impact by adding a 'reset' button to the device. This is a momentary off-on 1P1T push button which on command connects the #RESET Pin19 of the FT232R to earth and reboots the chip. This was a little tricky as - annoyingly - that particular pin is not broken-out on the SparkFun board I used, so I had to carefully lift it with a very fine lace-making pin and then solder a wire to the switch. Not easy at the scale of this IC, nor pretty! However I ended up securing things with the ever-reliable hot glue and the whole thing is hidden inside the box anyway. Now when it seems the SIO2PC-USB has lost connection with the PC I just need to push the reset button, effectively power-cycling the device and leading it to be detected again. This means I thankfully don't need to grope in the dark behind my PC every time this happens to un/plug the USB cable. So... Not problem solved at all, but certainly problem-all-but-ignored.
- A pretty solid little device now.