i do layers rather than all in one, layers are (below 10 deg) tee shirt & boxers, thin jersey top and shorts, hoody and joggers then zipped hoody and combats, 2xpair sox (1 x knee length boot sock) + in my bag i carry a thin waterproof jacket and trousers incase of rain but at any given time i can remove a layer or two.
if you have an all in one on you cant put much underneath, i found anything more than undies and you were soaked with sweat after ten mins, after you got warm you have nothing to take off unless you wanted to run round in your skidders