Re-opening of a grave

[Jonesy]

There are no hard and fast rules covering all cemeteries researchers must contact the cemetery or research the particular rules for the cemetery concerned.
Cheers
Guy

Good advice, Guy; I emailed Greenwich Cemetery. They very helpfully (and quickly) responded, confirming Gertrude Wills purchased the grave for Hilda Mason, and explained the purchase book confirmed that Gertrude purchased the grave "as a friend/companion of Hilda and also states that the grave was for 2 coffin burials".

[Peter Goodey]

as a friend/companion of Hilda

See my earlier suggestion about electoral registers and probate records.