Hi Claired
I checked the images of the register I have and found the entry for your great grandmother as follows:
Page Number: 14
Entry: 590
Name:  ALLAN, Jane R
Age:  37
Sex:  F
Date of Burial: 18 July 1909
Section: Non Conformist
Grave Number:  19773
Remarks: Private (reopened 15 Aug 1912)
Grave purchased by:
ALLAN, J. Address: 5th Ave. Melville, Johannesburg, on 29 May 1910
Because the grave was reopened I checked to see who was else was buried in the grave and found :
Page Number: 507
Entry: 1255
Name:  ROBERTSON, Marion
Age:  48
Sex:  F
Date of Burial: 15 Aug 1912
Section: Non Conformist
Grave Number:  19773
Best wishes
Shirley