In the novella Dead Man's Mirror, Hamborough St Mary is a village in Westshire. It’s the location of Hamborough Close, the home of Gervase Chevenix-Gore and his family. The nearest railway station is Whimperley and the nearest post office for telegrams is Hamborough St John. The village has a frutierer named "Carpenter and Sons".
The exact location of Hamborough St Mary is not stated. Westshire is not precisely indicative of a location in the west country of the UK. To reach Hamborough, Poirot was instructed to take the "four thirty from St Pancras" and instruct the guard to stop the train at Whimperley. This was not a regular stop but a special favour for guests of Gervase Chevenix-Gore. The train Poirot was on is recorded as "The Northern Express". The name of the train and the fact the St Pancras is used suggests a location north of London.