I thought there was no photograph of Annie in our archive but I was wrong. I stumbled across and annotated version of this photograph and to be surprise the rather rotond lady standing behind Tom's shoulder is clearly labelled 'mother'. She is wearing rather odd clothes and an extravagant hat. Looking into her life, she too left India and retired to England with a gentleman friend, so could well have appeared at the wedding.
When Tom writes to 'my dear mother' in his letters however it is clear from the surviving envelopes and the context that he is writing to Mary's mother, not his own.
Perhaps Annie just turned up, as can happen when an announcement appears in the Times! If you stare at a photo for long enough you can read all kinds of stories into it.