Greenwich Cemetery has vast areas of common graves (which may or may not be paupers graves). Many are unmarked, in areas with just grass. Some have small headstones, less than knee high, which characterise common graves everywhere. There's usually only one name per stone and although you sometimes see a whole row on a single grave, don't expect everyone to have one, so if you go to look, take a list of all 10 people. Contrary to what you might expect, common grave headstones seem more likely to carry plot numbers than those on private graves, which helps searching.
Section J is in the far left corner of the cemetery seen from the entrance. Follow the wide central road past the large war memorial area and it's the final section on the left, sloping down to the wall. Now comes the rub. Greenwich follows legislation promoting keeping areas of grass uncut to encourage wildlife. Many of the common grave areas are treated this way and I don't think I've ever seen J without knee-high grass, though an encouraging number of headstones can be made out peeping above it. The wildlife has a habit of digging deep holes, so it's unsafe to wade in. Notices say tne grass is cut in autumn, but I've rarely managed to hit the right time. As it happens, I too have relatives in section J and intend to have another go this winter. I'll see if I can find J/40, but no promises if or when I'll succeed. Please send me the list of 10 names by PM or e-mail. The cemetery closes quite early, 4pm Oct-Mar, which doesn't help.
As to wartime grave marking, my observation generally in London is that there is a higher tendancy for them to be unmarked or for engraved inscriptions to have been done so lightly as to be no longer readable. I guess relatives moved away and never got around to it and/or labour and materials were scarce. On the other hand, air raid victims buried in mass graves now usually have publicly-maintained war memorials with the names usually (but not always) better preserved and legible than on many private graves.