I found my husbands grandmothers grave at the weekend

after much searching for over a year, off we went to visit, it's an extremely large London cemetery. Armed with the map we found it, the headstone is as it is (except it is now dark grey in colour and doesn't look at all like white marble)
in a photo I found in my husbands mothers effects. The Headstone has the inscription for Grandmother, in front of the stone is a (large) urn with an inscription to grandfather, whose death I had been unable to find, despite sending for various death certs.
But underneath the inscription for grandmother is another inscription to an unknown person (different nationality- easily identified by name), I have no record anywhere in my research of this person I wondered is it a son in law?
So I phoned and checked with the Cemetery Office - definitely only two people in the grave they said no more room , are you sure?
Later I heard from them, the stone mason given the permit to add the inscription has added it to the wrong headstone, e.g. grandmother is in Section 57, grave 430
the other person is in Section 37 grave 430.
What to do? is there an obligation on the cemetery to correct the error, they don't know if they can find out who the mason was?
I was so upset...

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