In reply to Somerville:
You are supposed to be at the top of they Whymper by 8 in the morning in order to descend it. There is also a nerve racking knife-edge (and I REALLY mean knife-edge), snow crest from the summit to the top to of the Whymper, which is very imposing and requires sure cramponing.
Normally the Moine arrete is suggested as the late in the day descent, though that also starts with a steep snow crest, which will also have been in the sun all day. It is also quite complex and not obvious to follow in descent if you have not climbed it. There is said to be an abseil line on the Grande Rocheuse forming the right (East) side of the Whymper, but I do not know how to locate it.
There is quite a good argument for bivying on the very summit snow if you are late in the day, and waiting for early the next morning to descend.