The Sarah Siddons mentioned by Thomas refers to the 18th Century Welsh tragedienne, the foremost actress of her day.
Just to give a time frame of the series, a newspaper heading is shown dated July 1912. ....Also Geoffrey Whitehead's character mentions having had Charles Rolls(of Rolls-Royce fame) as a good friend. The real Charles Rolls was killed at Bournemouth in a plane crash in July 1910, two years to the month prior to this episode.