I am quite familiar with Streatham Cemetery and if you give me the names, dates, and plot numbers, here or by PM, I may be able to assess whether the grave can be found without need to involve the office. I might even have a photo.
If you go the PHOTOS tab on Find a Grave's main page for the cemetery. you'll find a photo of the plan on a big sign by the main gate. Click on "View original" (bottom right) to expand it in a new browser tab, and then on the map to see it at full resolution. This just shows the sections, though, not individual graves. The plot numbering is reasonaby logical, though sometimes you have to struggle to work out what that logic is! Most sections have the low numbers going around the first few rows near the paths, but in the middle may switch to running in parallel lines. There are a lot of war graves with their distinctive gravestones, and if you look at the listing on the CWGC page, you may find some near yours. But there's a lot of bare grass and some reused graves, which doesn't help.
Good luck!
