Very many thanks for your prompt reply and offer of help.
According to the memoirs of my great aunt, we had been led to believe that both my paternal GG grandparents had been killed at the siege of Cawnpore and that my G grandfather William Mason and his younger brother Robert had been saved by loyal servants who had hidden the two boys.
I have never been able to substantiate this story. Following a different line of research, however, it appears more likely that the boys' father John William Robert Mason died early in 1852 - possibly in Meerut. His wife, Sarah would have been expecting Robert at that time; he was born in July of that year in Allahabad. I think that Sarah must have re-married someone from within her deceased husband's regiment (1st Bengal European Fusiliers) - a Robert James Reid (or Reed), which would explain her presence at that time in Allahabad. The regiment moved to Cawnpore in 1856 but had moved on by the time of the siege, but it is possible that Sarah died whilst at Cawnpore and that her new husband was unable or unwilling to raise the two young boys, so arranged for them to be left at the military orphanage in Agra.
Any help with establishing the true facts would be appreciated.
Very best wishes,