As Paul says it is indeed now. Although perhaps late May, early June (usually referred to as duffers fortnight!) is actually the easiest...even I catch a few then!).
Different rivers are best at different times, but if they have a May Fly hatch then when ever that is is the absolute best time for catching.
However, what I am sure Paul is referring to, is that now is one of the nicest times to be on the river. Particularity those small feeder rivers onto which you never venture as a Barbel man, These rivers are beautiful in spring and easiest to fish as the trees are not so overgrown that every fly ends up in one rather than on the water!
Stalking a rising fish, on a little stream. Casting a fly to it..this is just what fishing is all about to me.
I was out yesterday and only managed one fish during a very brief lunch time hatch. The fish was about 8 " long as a beautiful, wild brown trout...without a name, never seen a pond and never been caught before.
I then sat and watched dippers catching flies out on a rock I had been fishing from.
I like to travel light and to move around when Barbel fishing and this is even better with trout. A 7' 6" 3 wt rod, a box of Gray dusters, a net and that is about it.
Fish spotting and learning fish behaviour are all parts of this and all transferable to Barbel angling. I am always surprised to find so many Barbel anglers deserting the rivers this time of year...not me.