Installing cable underground

[Anthony]

Got V+ (best thing ever) and about to ask VM to install an additional box into the conservatory, which I think is an extra fiver a month + install fee.

My question is about the new cable that will need to be run - round the outside of the conservatory is a small trench about 12 inches deep by about 6 inches wide with little drainage stones in it. Ideally i'd like Mr Virgin Installer to run the cable 'underground' in these stones rather than tack to the wall, is it likely he will do this?

a) can you bury this type of cable?
b) if I need to install some sort of tubing with a draw wire ahead of time then what type of trunking or tubing should I buy?

Thanks