Fitment of the cruise control buttons is very simple, the wiring is already there.
As Mo has said above the price from Volvo was around £400 just for the software upload! They have to protect their aftermarket.... Believe it or not, it's basically a simple "on/off" software switch! With the software not loaded, the VCT and Vadis software will read the status of those buttons quite happily!
Braydons Volvo
http://www.braydonmotors.co.uk/
Looks like their site is being redeveloped, but they are very good
IIRC the price from Braydons using their kit to enable the functionality was £100 .