Hello Shelly
I was able to visit the church last week on a family visit to Liverpool (I live in Brighton now) as we too have just discovered our ancestor William Japha is buried in the graveyard (in 1891 conincidently), and like you, we have a plot number.
However, it looks like none of the stones are in situ (apart from the few large memorial structures) and everything looks to have been moved about following a bombing in WW2 and renovation. The stones are now flat, lined up next to each other in rows, and in no particular order (or so it looks). Many are covered by earth or eroded.
I haven't been able to find an original map that would allow us both to see the vicinity of the original placement and have asked/emailed the verger as suggested but with no response yet.
My brother has visited several times trying to see someone at the church but is is always closed when he goes. I have this week found out there is a 'Friends of St.Marys graveyard' who have been cleaning up the area (and have done a brilliant job as my brother said there is a vast improvement since he first visited late last year) and I will be contacting them to see if they can help us. there some excellent sources provided in another link that I've just started on the church (In 'General Queries'
).
Good luck
Denis