Answering my own question:
I updated the drivers for the undefined USB device by looking in the Drivers folder (not the FTDI folder, one up from that). This resulted in the USB interface becoming defined as an enumerated Com port and no other driver installation was required. I have just uploaded a sketch to the Mega and it all seems to be working.
I couldn't find anywhere in the documentation that covers this method. Hopefully this might be of use to someone else who has the same problem.
John.