A Vision of Britain through Time states that Runcton Holme was a hamlet and chapelry before becoming a civil parish. It also states that it ceased to be a civil parish in 1935. There is a another entry in the same database with the standard "AP/CP" (ancient parish/civil parish) description. Wikipedia considers it a civil parish since 1974. The writer thinks that A Vision of Britain through Time is somewhat incorrect here. It would be possible for a place which was a hamlet and chapelry to end "early" (usually before 1800), but civil parishes did not become common until 1866. It is possible that the description "CP/Ch/Hmlt" should have been just "Ch/Hmlt".
Other notes in the database mention that Runcton Holme expanded in 1935 to include the parish of Wallington-cum-Thorpland as well as South Runcton. This confirms the article from William White's History, Gazetteer, and Directory of Norfolk 1845 (reproduced by GENUKI) which states that the church at Runcton Holme (dedicated to St. James) was consolidated with South Runcton and Wallington as a united rectory before 1831.
An outline map of Norfolk parishes dated 1917 shows the parishes of Runcton Holme and South Runcton sitting above the parish of Wallington with Thorpland.The Ordnance Survey Map of Norfolk revised 1944 replaces the three parishes with the one parish of South Runcton.
Wikipedia provides an up-to-date description: Runcton Holme is a civil parish in the English county of Norfolk. It covers an area of 13.89 km2 (5.36 sq mi) and had a population of 676 in 288 households at the 2001 UK census, the population reducing to 657 at the 2011 UK census. For the purposes of local government, it falls within the District of King's Lynn and West Norfolk.