Menai is 300k, Dolgellau is 400k.
I've just posted off an entry, which is a change of plan as I was going to ride the ride the Brimstone. However, that would have meant about £70 in accommodation and £40 in fuel.
I will probably try and book some accommodation somewhere near Beddgelert on the return and try and batter it up and back, then rise early to get to Dolgellau before closing. Don't care what time I finish, but it always seems to be 5.30pm regardless of how slow/fast I go.