Martin P-L wrote:

Roy Atherton (Peter) 1915-53

Brought up with his cousins by Reginald Cecil and Lilian F.  Attended Ladycross prep. school, then Pangbourne naval college.  He became a Captain in the Royal Marines.
Married Eileen.
He was killed in a car accident in Ceylon, and buried in the military cemetery there.  At the time he was Deputy Commander of the Royal Naval Police.
Eileen lives near Bristol, writing and painting a bit.  TP-L, when he was at Bristol university, saw some paintings of hers exhibited and sought her out.  I, too, have met her, but her only regular family contact has been with Olga and one of her daughters.
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On 15 Jan 1998 AP-L had his birth certificate, given to me on 16th May 2000:

Registry No 216 of the Putney sub-district of the Wandsworth district of London
Registrar: Amelia Hudson
DOB: 1st July 1914
Place of Birth: 131 Upper Richmond Road
Name: Roy Atherton
Father: Richard Barrington Powys-Lybbe, of independent means, of 38 Palewell Street, East Sheen
Mother: May Mathilde P-L formerly Rawstorne-Frye
Registered on: 12th August 1914

From Eileen, his wife:

"Peter finished the war as Captain Royal Marines.  They fought the rear guard action in Crete, against the first parachute air born drop by the Germans.  He was evacuated in HMS Dido to Ceylon.
"After the was he went to Ceylon as Deputy Officer in Command of Royal Naval Police, Ceylon.  I joined him in 1947.  He was killed in a car accident, on the jungle road from Colombo to Trincomalee, - where the Naval base was, & where we lived.  He is buried out there 1953 aged 38.  No children."

TFPL, 2010: Steve Rogers of The War Graves Photographic Project sent me a photo of his gravestone on which is inscribed:

          CAPTAIN
     R. A. POWYS-LYBBE
       ROYAL MARINES
      1ST MARCH 1953

So I have included ‘Captain’ on this database.
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TFPL, March 2003: I have seen his entry in the record books of the College of Arms.
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Sept 2009, TFPL: I have applied for his Naval records from:

b. Commissioned officers who entered the service between 1924 and 1939, inclusive.
Navy Records Office
TNT Archives Services
Tetron Point
William Nadin Way
Swaddlincote
South Derbyshire
DE11 0BB
Telephone: 01283 227913
FAX: 01283 227942
navysearchpgrc@tnt.co.uk
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From the London gazette:

20 Apr 1945: Temp Lt (Actg Temp Capt) R A Powys-Lybbe to relinquish the rank of Actng temp Capt.

6 Nov 1945: Tempt Lt to be Actg Temp Capt.
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TFPL, Aug 2016: In the 1939 Register he was living with his two maiden aunts Sophia E and Mabel Frye plus two redacted youngsters (whom?).  He was male, single, born in 1 Jul 1914 and working as Overseer and Secretary of Factories Callender ??? (Contractors)
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From the London Probate Index:

POWYS-LYBBE Roy Atherton of Cannonball Cottage Fort Frederick Trincomalee Ceylon died 31 March 1953 on the Trimcomalee-Anuradhapura road, Trincomalee Probate London 4 August to John Brett Fletcher solicitor and sir Charles Russell baronet.  Effects £318 1s 11d in England.
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Their marriage on FreeBMD:

Surname   First name(s)   Mother/Spouse/Age   District   Vol  Page
Marriages Dec 1940   (>99%)
Longden Dora E Pavys-Lybbe Portsmouth 2b 1679
Lybbe Roy A P Longden Portsmouth 2b 1679
Powys-Lybbe Roy A Longden Portsmouth 2b 1679