Finally managed to locate a free day-time skybox generator and made a sky box for my game.
I lifted the code from my space game to add it to the platform game. Basically creating a cube map using the six faces of the texture, then generating a sky box geometry mesh using the position-only vertex format and a special vertex and pixel shader for doing the rendering, moving the pixels to the far plane and sampling using the interpolated normal. Nothing very clever but it works nicely.
Can soon make the image used be a property of the level in the editor easily enough and play around with tweaking the colours depending on the time of day and so on. Could even do a blend between a night time and day time cube map easily enough if I wanted this to be dynamic, but no need to complicate things at the moment. Need to instead focus on getting the main character interaction with the environment working as I want it.
Trying to decide on a nice balance between a Mario-style running, jumping interaction and a Tomb Raider walk-run-jump mechanic. Still not sure what kind of game I am trying to write here.
I have walking and stopping at edges working again now, but don't want to stray too far from a Super Mario free running and jumping style so not sure that will be kept. Need to knuckle down to some hard decisions about the type of platform game I'm writing here.