Thanks for the responses guys. To be honest after that 1st print I didn't focus on the issue I saw on the foliage picture as the zebra stripes were so evident on the 2nd and all subsequent images. After I printed the 1st picture though, realising that I had chosen the wrong paper type, I simply recalibrated and created a new profile using the Photo Semigloss paper setting. In both the cal and profile patterna I didn't see anything that suggested either head or ink cartridge problems. I've also done an alignment check and a paper feed check and those patterns look good as well, with very fine lines showing no signs of drop out. On the prints and test print sections I've made since the 1st one I've not seen the flat grey issue come back with the correct profile, but I've not reprinted that image.
So I suppose I'm kind of dismissing this issue for the time being until I can see the stripe problem resolved which, on my printer and apparently on the printers in the HP Tech Support area, is pretty pervasive. By the way to answer Christopher, all of these prints were made on a 24" roll of HM FAP. The only other print I made was on an Epson Premium Lustre roll (17") to determine if I may have a bad roll of FAP. I mentioned above that I got slight banding on that print, but if the heads have been hitting the paper I'm not surprised something might now be slightly out fo whack. I've not done any more alignment/calibration tests since then. For regular banding issues the troubleshooting guide that is installed with the printer software says to do a head clean, alignment check, paper feed check in that order, so I would start from there if I felt like playing some more.
I don't think I'll be doing any more tests, at least until I speak to HP Engineering next Tuesday.