I have three pairs of black M&S trousers, bought for work (actually one pair is navy but so dark I didn't realise for several weeks. I wore them to work for at least one winter before I retired and I have worn them regularly every winter since and they are still going strong! Originally bootleg style I did taper the legs a few years ago.
I still have several jackets, though really I should cull some of them, and a couple of dark skirts, I'm wearing one of them today. As several people have said, it's mix and match the 'smart' stuff with less formal/more comfortable casual items.