This morning I saw the sun shining on the catkins of this tree outside my study window and I had to get my camera out.  I spotted a dark brown butterfly nearby so adjusted my settings to try and catch one in flight. I didn't see that one again, but this butterfly, flew towards the upper branches and alighted on the catkins.

I think it is a Large White (Pieris brassicae) and it appears to be quite common and not a threatened species.  I assume it has hibernated and come out with the warmth to prepare to lay its eggs as soon as possible, probably on cabbages.

I managed to track its flight and am pleased to have caught it on the wind.  At one point a bee attacked it when it had alighted on a bud and it flew away for a few inches, and its flapping wings seemed to get rid of the bee and then it returned to the same spot.