Well, it didn't, I wandered and watched, tracked duns downstream until they were out of sight and never saw one taken.
I eventually gave in and decided it was time for a change, with this amount of activity the fish had to to feeding somewhere, and if it's not at the surface....
I opted for a pair of spiders and an olive nymph on a ten foot six inch (3.2m) leader - three feet (0.91m) between spiders and four foot six (1.37m) to the slightly weighted nymph - bingo !! I had a fish to the nymph on my first drift.
A few more fish followed, but none to the spiders, all came to the slightly weighted olive nymph on the point.
We all have our theories on why things do or don't happen, today it may have been too bright on occasions, but there were plenty of cloudy intervals too. It might have been down to water temperature, the fish may be very lethargic and not prepared to work (come to the surface) for their food - but there's always a chance as long as you're prepared to ring the changes, the fish are there and sometimes it's just a case of finding out what they want and where they want it.
The long range weather forecast suggests that things are going to warm up, so hopefully we should be seeing a bit more activity both at the surface and below in the coming weeks - bring it on !