Ok so I've looked around and found a dealer that does the following electrics:
13 Pin wiring kit 31414907 £38.41
Trailer module 31407108 £52.51
Then I would just need the software which I believe can be done using my grey-market DICE & a cheap (£27) one day VIDA subscription from
http://tis.volvocars.biz/ and buy the towing module software (£20). Although I haven't done this before so it would mean more time setting up and learning the process which from what I have read means buying the update from a Volvo dealer and using VIDA to find/download and install the purchased software.
Hardly seems worth it unless there are any other software updates that are worth having?