This might not seem much, but it's a big deal, really! Look, see that road overlay mesh, it has snow on it - real, proper in-game snow - and shadows no longer clip through it - working shadows, I tell you!
OK, the colour/lighting is still not right, but I am getting there.
So pleased with myself right now! :D
(Thanks have to go to ja_viera, who had already covered at least 50% of the ground in solving this, and whose work I poked, prodded and pulled apart so that I could get this far.)