Hi Peter
According to Chris Reynolds on his Genealogy in Hertfordshire site who is a very reliable source.
http://www.hertfordshire-genealogy.co.uk/index.htmElstree (or Idlestree) is a village, in Cashio hundred, Barnet Union and county court district, rural deanery of Watford, archdeaconry of St Albans, diocese of Rochester, and North-Western Metropolitan postal district, 5½ miles north from Harrow railway station, 5 south-west from Barnet, and 11 from London, through Edgware, situated at the intersection of the road from London to St Albans (the ancient Watling Street) with that from Bushey to Barnet; the later road divides the county of Herts from Middlesex: the four parts cut off by these roads are in four different parishes and two counties;
the north-eastern portion, in which is the church, is in Elstree parish and Herts county; the north-western in Aldenham, Herts; the south-western in Little Stanmore, Middlesex, and the south-eastern in Edgware, Middlesex. The church of St Nicholas is a handsome building, with tower and spire and 2 bells, having been rebuilt by private subscription. The living is a rectory, annual value £804, arising from 200 acres of glebe, with residence, in the gift of the Lord Chancellor, and held by the Rev. Charles John Plumer, M.A., of St John's College, Oxford. There is a National school. The area is 1370 acres, and the population in 1861 was 402.
Thus if Thomas lived in the north eastern portion near the church he was in Herts.
Rick