In reply to Phil1919:
So whatever you do you're out of luck.
I've been looking at this recently as I want to do the Welsh 3000's (maybe in May). Tried it with a group a couple of years ago, (intentionally over two days as we were of mixed fitness) and we decided to sack the second day off as the two drivers felt that they wouldn't be able to drive back to London after a full day in the hills.
So next trip I'm thinking we come by train, but the buses are putting a bit of a spanner in the works. Best plan I have so far is to get to Pen Y Pass on Friday afternoon and bivvy on Snowdon.
If anybody has a good source of historic summit temperatures that would be useful. My searches came up short.