Mike
Thank you, yes I am extremely pleased to say it worked.
Firstly I updated El Capitan.
I disconnected the usb lead from the printer.
I restarted the iMac (after installation). Reconnected the printer. This time everything seemed to work. It was the procedure that seemed to be key. I cannot recall exactly what i was doing previously and why it hadn't worked, but I suspect the updating of OS 10.11.3 to the latest version 10.11.4 was pivotal.
Thank you for your assistance (and I am pleased if my experience and the support I received to understand fully the help menu might go on to help others, since I discovered hundred of users had had similar printer problems to me).