Hello Cakeworld, does anyone know if/how the PC running Sonar can recognise outboard modules like this one which has a selection switch for 'PC Host' and 8pin DIN socket? Thanks

Based on another similar post/question and my faded memory, I think that host jack connects to a serial port on your PC... The problem is, serial ports haven't been available on PCs for awhile.
 
Of course, Sonar can communicate with the module via the MIDI in and outs... Are you looking for more than that?

Hello JonD, thanks for the response. I am trying to use the PC Host connection from the RS232 port (which the W7 PC confirms as working with the relevant drivers) without using the Midi in/out (yet). A generic Yamaha module driver is also correctly installed in the PC, it is just that there is no sign from the PC that the two are linked. Thanks, John

For what it's worth, the USB PC Host port only works for me on this computer (Win 10 64-bit) while using the Steinberg/Yamaha ASIO driver and my Yamaha MX 49.
On my DAW (Win XP 32-bit) the computer doesn't see the Yamaha USB. I don't have the Steinberg/Yamaha ASIO driver installed there.

Personally, what you might want to do is grab your self a recent USB to MIDI converter.  That way, if you ever decide to upgrade to Windows 10, you are less likely to have to be worried about finding drivers.

What I have done here is installed an SW1000XG (which is similar to an mu80, but on a PCI card) installed in a 2nd Windows 7 32bit machine (an HP SFF).  I use a Roland UM one Mark II USB to MIDI converter to play MIDIs from Sonar Platinum on a Windows 10 64bit PC. This scenario should also work in your case as well, except substitute the PC for your MU80. Te MIDIs I sequence, as well as your standard controller events,  contain plenty of sysex commands as well.  This set up does not miss a beat.

+1 for the USB MIDI converter.  Or if you have an audio card with a MIDI-In, use that.  I have a Roland SC-55 (the MU-80 Roland equivalent...sort of) permanently plugged into my PC with a USB MIDI plug.  When I use it with SONAR, I select the MIDI-In and point it to either standard MIDI to use voices 1-128, or Roland GS if I want to access the other sounds the unit has.  You would do the same, but instead of Roland GS, you would select Yamaha XG. 
 
I've been jonesing for an MU-80 lately myself...!

I had an MU80 and traded it up for an MU100. I use it with a Motu microlite which provides a 5 port interface. You can get these very cheep on eBay, Reverb, and other sites used - but they're actually quite rugged.