Sorry for the confusion and I really appreciate folks trying to help on this one. It's a hard one to explain.
Ok, the dialogue that comes up when I print from Apple software (e.g., Safari, Preview, Mail etc) is attached. THIS IS THE ONE THAT DOES NOT WORK. You can see that I have Plain paper set as the Media type and Ink is set to Matte Black.
When I hit Print from this dialogue set thusly, the p800, WHICH IS SET TO MK, does its thing for a few seconds then beeps and then gives me the attached Notice asking to change to black ink type specified by the printer driver.
As the printer is set to MK and the driver is set to MK, this notice does not make sense.
For the record, I have the newest printer drivers, have reset my printing system and re-added the printer, all to no avail.
By the way, failure to print as described above occurs regardless of the Media type selected (I had originally thought it had something to do with printing on Plain paper).