So I've popped out and bought:
a) a simple black skirt from COS, made special by the large bow in front. It'll work well with a tee and gladiator sandals for day, and maybe higher shoes for evening (although I'm a flats girl at heart).
c) Also these shorts( from Topshop as well):
They're part shorts, part skirt - great with tights for London in April, and even better with bare legs in summer.
The thing with shorts is that on an Amazonian model with legs up to there, or a pre-pubescent teen with minimal body fat, it's pretty much a no-brainer.
If however, you're part of the other 98% of the population, with legs that aren't a metre long, they may take some effort.
I recommend opaque black tights for as long as the chill lingers in the air, and a hefty dose of swagger when it warms up.