In reply to Stani49:
Assuming you're starting on Snowdon the only problem is the Carneddau. I took sterilising drops and a 1l bottle, filled up in Cwm Glas Mawr, Cwm Dudodyn (under Elidir), Llyn-y-Cwn (this is the one I wanted the steritabs for), outflow of Llyn Bochlwyd and then drank a shed load at Ogwen Caff and filled up there. That was the last fill until the end (except for a tot of whisky at Foel Fras
).
I didn't do the traverse to Yr Elen so don't know about that spring.
If I did it again I'd do exactly the same except I'd buy an extra litre at Ogwen to keep me going over the carneddau.
Of course many people don't go to Ogwen caff preferring to follow the Afon Lloer and east ridge of Pen yr Ole Wen and thus avoiding the hellish grunt up it from Pont Pen-y-Benglog. I was totally unsupported though so really needed to stock up. An alternative would be a stash somewhere near Glan Dena