Following help from Matthew three likely assembly areas have been found within about a mile of each other here in Odiham, North Hampshire. Two are on the edge of small areas of woodland but the third is especially interesting as it doesn't appear to be associated directly with woods. It seems possible that the males may be coming from sallows on the edge of a main road, adjacent hedges or even local gardens. Clearly Purple potential in the landscape generally is greatly enhanced if woodland is not a necessary factor for the success of the species.
From personal experience I'd urge everyone to check in and around local high spots, whether or not they feel promising at first. I'd lived here fifteen years before I realised that the butterfly was strong locally, so even allowing for my poor observation skills it's amazingly easy to miss.
It's just possible that Iris can adapt to open and semi-urban situations much more readily than we currently realise.