Hi
could somebody give a newbie advice on how to create an overcast day, grey sky and clouds to suit a snowing scene?
Easiest way would be get this:
and choose on of the presets 
( I don’t think you can make the default sky look very moody… )
A start would be to change the Rayleigh scattering color in the SkyAtmosphere component of the SunSky actor to a grey (between 0.1 and 0.5 or so for each RGB). Modify Exponential Height Fog to obscure the horizon by decreasing the Height Falloff (which increases the transition zone of fog from its lowest level into the sky/air above). Probably need to get a different set of clouds than the defaults because an overcast sky’s clouds are usually different, though it could be a cool effect depending on the scene, with the defaults. There’s other settings to modify in EHF and SkyAtmosphere (Mie scattering and Atmosphere Absorption) that could make it somewhat more well-defined as overcast and cloudy. Explore it if you don’t find a more helpful guide.