To complicate it further, there is a small rectangular Z3C section between Z7 and Z12 on the corner of High View Road and St Lukes Path (sic; their apostrophe key must be stuck). However on what I think is the earliest map I've seen (from the CWGC site), it's labelled Z9, so this may not be applicable to early 1900s burials. Worth checking, though, if all else fails, as also is Section 3Z in the St Pancras part of the combined cemetery.
I avoided giving detailed location information before, partly because of the complications, partly out of laziness, and partly because on previous occasions I've been pipped at the post by someone else (once after I'd spent some time weeding a grave for its photograph). But if anyone who's close enough to get there in the brief sunny intervals between the deluges wishes to do so, please let me know. Rain is currently predicted for this weekend too.