ambleetc,
The Northumberland County Farm Index database ( it lists Northumberland's communities of all sizes mid 1800s within their appropriate townships and parishes)
It lists " Plainfield" as lying within the "township" of Flotterton, within the parish of Rothbury.
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Genuki web site from 1868 gazetteer shows...
FLOTTERTON
a township in the parish of Rothbury, 4 miles west of Rothbury. At Plainfield
in this township, Lord Derwentwater and others first raised the standard of the Pretender ( " Bonnie Prince Charlie" ) in England.
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Flotterton was one of the 28 townships that made up the parish of Rothbury.
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See Greenwood's map of 1828
http://communities.northumberland.gov.uk/007482FS.htm Plainfield, north of Flotterton, just sneaks onto the left hand side of the map.
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Godrey Watson, in his book " "Goodwife Hot", Northumberland's Past in it's Place Names" in writing of Scandinavian place names around Rothbury, says
" .... Some like Plainfield have been anglicised, the name denoting an Expanse of land shaped like an Arrow, ends with the Old English " feld" , meaning a piece of Open Country, more extensive than a Clearing. Yet it begins with the Scandanavian word " fleinn" and indeed as recently as 1572 the place was known as Flaynefeld.
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Today there is a farm , Plainfield Farm at Plainfield. ( postcode NE65 7LE).
Looks to be the only building(s) in this very rural area.
The old map for c1865 shows an identical situation.
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Michael