Neil,
I have had great days using dries on Loch Insch, Artloch and, of course, Haddo. From my limited experience, I think that it is still too early in the year to be trying dries but others may dispute this. I was out at Waulkmill this morning and the water is still very cold and, although the martins/swallows were taking something in the air above the loch, there was very little surface activity from the fish.
My own personal favourite dry flies are tied with CDC using the Marc Petitjean method ( examples of these can be found on YouTube ) and also Klinkhammers.
As to the best time of day - anytime the fish are feeding on the surface!
I hope this of some help
Peter