Hi WestOz,
Welcome to Rootschat...
To help you visualise what Chris has explained, and for your general interest/records, the updated map that Chris mentioned can be seen via this link from the website of Islington & Camden Cemetery Services (ICCS):
https://iccslondon.co.uk/app/uploads/2017/05/ST-pancras-Site-Map-updated.pdfYou might need to enlarge the map (by pressing the + tool just above it) to see the area labels because they're pretty small, especially the one for Z3A.
If you locate the crematorium (on the right-hand side of the map) then look just south of that, you'll see an area labelled Z3B which, as Chris said, is alongside the road (High View Road). Just south of that, you'll find Z4A and Z4B; and just south of Z4A, on the other side of the road (Raleigh Road, which turns off from High View Road/Parkers Road*), you'll see a relatively small area with a tiny label identifying it as Z3A. (Again, right next to the road, as Chris pointed out.)
* I think they might have skipped an apostrophe - maybe it should be Parker's rather than "Parkers".
I do hope your ancestors' grave memorial is still there, and easily accessible, to make the task straightforward for Chris, and to ensure a positive outcome for you.
Best wishes,
theirchild
PS: Well done, Chris - you're an absolute star for offering to visit the cemetery on WestOz's behalf!