20 May 2018
HP OfficeJet Pro 8715 Printer was suffering same problem as outlined here:
BlurryDoublePrinting . . print head replacement
https://h30434.www3.hp.com/t5/Inkjet...
It suggested a new "Print Head" would be despatched by HP.

Chat support with HP suggested:
"“2:17 PM Samson: You have to press the following combination of buttons
Home icon return Key Home icon Home icon
These are offscreen buttons.
You will get manufacturing Menu.
Scroll this menu to find service menu
In service menu select service test
In service test, select printhead recovery level 3”"

Followed these instructions, although they were not entirely accurate.