In reply to DaveHK:
Thanks Dave, I had considered that, although I didn't want to reach the summit too early (in terms of visible light) as I wanted to just be able to turn and attack Crib Goch. I've been on Snowdon a few times now, and done Crib Goch once, but these were both clear sunny days. Cirb Goch in the dark is a no-no, so the level of light between Snowdon and the pass of Llanberis is critical, I reckon.
Post edited at 11:36