I now need to revise my prediction, backwards. This is because none of my captive larvae have pupated, though they are ready to do so, and also because none seem to have pupated in the wild. Yesterday I managed to find four wild larvae, two of which were full to bursting point and turning pale, as they do prior to pupation. The other two were almost full fed. The forecast for the coming week, which is not a high confidence forecast, suggests that they should start pupating this week.
At this stage, it looks as if iris will not be out anywhere in June this year, though it should begin at the start of July at early sites such as Bookham Common. Elsewhere, it may not appear before July 7th at the earliest, more likely the 10th.
My advice is to book your hols for the second and third week of July.
Meanwhile, here's a game of Spot The Iris. There are two full grown larvae in the picture, taken yesterday near Savernake. See if you can spot them -