Thank you for this. Do you know the full title of Robert Reid's regiment? I had been working on the assumption that Sarah would have married another soldier from her deceased husband John's regiment - i.e. 1st European Bengal Fusiliers. I have researched the movements of the Fusilier regiment so I know they were not present at the siege of Cawnpore in 1857, which is why I think my great aunt's story can't be true. However, from the evidence of Robert Mason Reid's birth record, it looks like I now need to research the movements of the 10th Regt N.I. over the period 1852 - 1857.
By the way, I have found a marriage of Robert Mason to Ann Polson in 1876, but other than that he seems to have vanished without trace. According to my great aunt, he became a tea planter but I don't have any evidence for this. It may be that he used the surname Mason Reid rather than just plain Mason.