In reply to twm.bwen:
There is a black and white photo topo showing Mallory's Slab in "Rock Climbing in Snowdonia" by Paul Williams, but it doesn't go to the granularity of showing individual pitches. I remember starting on MS about 15 years ago in plastics, to practise for the Alps. Having travelled 30 feet without any gear apart from an old peg, I couldn't wait to retreat. I managed a return on Thursday, but went for Avalanche/Red Wall/Longland's Continuation because we had the Rockfax guide with it's full colour photo topo. However, apart from the places where you start Red Wall and Longland's Continuation, I could possibly have followed the polish. That's not to say it's badly polished but the virgin rock in general is quite sharp, so any smoothness is obvious to the touch and re-assuring in a someone's been there before kind of way. That said, it is unlikely that Mallory's Slab has had anywhere near the same amount of traffic. It might be worth going for ARWLC instead if you've not done it before, as there is always the scramble available on the left at the half way point if things aren't going to plan. You have to bear in mind though, that I'm saying this with no knowledge of your parties abilities, ARWLC is very committing for its grade beyond half way.
Anyway, hope you have as good a time as we did, whatever route you choose. In my opinion Lliwedd is a jewel in the crown of Snowdonia.