Just to update:
Volvo towing electronics (Control module, loom extension and 13Pin plug assembly) arrived on Saturday and was quite straightforward to fit. The most fiddly thing I found was getting the fuse block out of the fuse holder to enable to feed the orange-wire through the block before completeting the circuit with the pre-exiting wiring by inserting the supplied fuse. Apart from that it's just some plug-in connectors and lots of cable-tieing to keep things need and tidy.
Plugged in trailerboard and no lights which did have me slightly concerned i'd not connected something up correctly as I thought it might have given a basic output without dealersoftware.
Software was installed by main-dealer today and towing electrics sprung to life immediately, so all is good