Not on the 1.2 - it makes no difference.
The 1.2 500 went from 119g/km to 113g/km after fitting S/S, but interestingly the Mk3 Panda went from 119g/km to 113g/km just by changing to the Euro5 engine (which is the same as the 1.2 500), without needing S/S.
@
jrkitching I'm a bit surprised tbh! The S&S in your car has worked flawlessly hasn't it? Tbh I think this is proof that a properly maintained car driven by someone who knows how to drive a car (don't hold the clutch in needlessly, rest your hand on the gear lever and so on.....)
S/S is a nice to have. I use it, appreciate having it, and I certainly wouldn't deliberately disable it. It makes driving in town a bit more comfortable, but I'm far from convinced it saves any appreciable fuel unless all your driving is in a big city.
If given the choice between S/S & £300 off the car, I'd take the money, but then again, I'm a tight fisted old git

.
If you have it and it's not working, keep badgering the dealership until it's properly fixed. I don't buy into the argument that most of the issues are down to driver error; more likely the fitters either don't know how, or can't be bothered, to set the cars up correctly before delivering them to the customer

.