Hello, everyone. Gypsy, I'd also like to say thank you for the tip about copper. I'd never heard that before and, while I don't like to use slug pellets, I didn't know what else to do, though I had heard that wood ashes work to some extent.
Peg, the best thing to do---though I know it's lots of work---is to wait until late June or so, dig up all the iris crowns, divide them and then dig up the entire flower bed before re-planting. Dividing the rhizomes will re-vitalise your irises and they will bloom better. Also, they will probably appreciate a dose of fertiliser that has an NPK balance of about 6-8-12 or 5-10-10 both after their blooming period and then again in March.
Thanks once again everyone.