There's no way it only cost £15 in total to install a smart meter. The cost of the meter has to be at least that much and then you have paying a guy to actually install it (which in my case took 3 attempts and caused a gas leak, which took 4 or 5 guys several hours to fix - although hopefully that's unusual).
15 pounds cost per year, 11 pounds saving per year, for an average user.
Not sure how many years the cost is assumed to be spread over and what the ongoing running costs are but that's the per average customer cost apparently.
