Alastair, thankyou for updating us with this information and I'm sorry to hear that the problem is indeed a virus. Viral infections in Irises is not something that there is much information about (especially on this forum), as opposed to other genera I grow (such as Liliums and Orchids, where there is somewhat greater awareness of viruses, although often still poor hygiene when it comes to Orchids).
Based on your experience, I will be checking to see what my testing options are here in case I ever need to get some testing done.
