Actually looks like a profile problem not a hardware problem. I fired up another 3880 which had been lying dormant for a while (I hadn't printed on it for at least 6 months and although it went through a cleaning cycle when turned on, the nozzle check disclosed no broken lines at all). Exactly the same result. And again a purge file prints pure yellow. Checking other prints against my monitor, I would say that the monitor is pretty well calibrated. I have also checked all printer driver boxes etc. and am clear that these are as they should be - i.e. no colour controls other than the profile through LR.
It is curious however. I have detected no problems with "normal" prints which include various shades of yellow. But these 2 photographs were pretty much pure yellow, and, for what it is worth, reported to be within gamut in Photoshop. But the prints are muddy and orange and completely at odds with the image on the screen. Out of curiosity I printed another version using Printer Managed Color with the Color Mode in the driver set to Adobe RGB and with Epson Color Controls. The result was dramatically worse than the versions with the profiles.
There seems to be some problem in the interaction between my profiles (canned and home made with Spyder print), Ilford Gold Fibre Silk and the Ultrachrome ink set. I remember being horrified by some prints I made a couple of years ago of yellow flowers which turned out to be orange, but when viewed 24 hours later in daylight (not the halogen in my room at night) were OK. These two show no difference the next day and in daylight.
Maybe time to buy a more sophisticated profile.