He was first known of from a letter from his mother to the war office in April 1918.  A search of 1837 OnLine showed that there was a record of his birth as Philip P-L in the quarter Oct to Dec 1916, that his mother's maiden name was Fry and the Registry reference was Richmond B., 2a, 855.

He was obviously still alive in April 1918 so I searched the 1837 online deaths indices from then until Dec 1925.  There was no entry for his death in that period.

The obvious thought is that he went with his mother when she was divorced by Dick P-L and then she married the co-respondent, Ernest MacLeod.  Possibly Philip then changed his name, formally or informally to McLeod.  But it would be interesting to find what happened to him, not to mention whether he had any children.

2009: I searched through all death index pages from 1940 to 1983 and found no death of a Philip McLeod.
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I wonder if this 1988 death registration is relevant:

Name Registration District County Year of Registration  Month of Registration
MCLEOD, Philip Chichester West Sussex 1988 May

Birth date: 19 Nov 1916

Registration Vol: 18, page 1310, Number: 588

The birth date is remarkably similar to the index of his original registration as Philip P-L.  It looks like the birth and death certificates are needed
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Possible marriages:

(a) Jan - Mar 1943: Philip H McLeod, m. Tedaldi in Battersea dist, vol 1d, p. 445
(b) Jul - Sep 1943: Philip McLeod m. Pendred in Holborn dist, vol 1b, p. 774

The spouses of these marriages were:
(a) Angiolina M. Tedaldi, otherwise the same
(b) Joan Pendred, otherwise the same. The cert for this has been ordered.

Less likely marriages:

1968, Q3  Philip A McLeod  m. Bailey in Bromley dist  vol 5A, p. 1622 --- Not found
1972, Q1  Philip C McLeod m. Marion Dwyer in Wandsworth dist, vol 5E, p.833
1872, Q1  Philip S MacLeod m. Rosemary J L FOOK or LAI-FOOk in Chelsea dist, vol 5A, p. 1332 (twice)
1975, Q3  Philip T McLeod m. Joan ORCHARD in Basingstoke dist, vol 20, p. 0306
1977, Q4  Philip A McLeod m. Christine A CHAPMAN in Swindon dist, vol 23, p. 1395
1979, Q2  Philip McLeod m. Amanda E CANNING in Thanet dist, vol 16, p. 1463
1981, Q2  Philip A McLeod m. Catherine G L CONNAH in Swindon dist, vol 23, p. 2158

and:

Name Registration District County Year of Registration  Month of Registration

MCLEOD, Philip W  Maidstone, Kent  1984  May  m. Jacqueline R Maynard, vol 16, p. 1187, Reg No 584
MCLEOD, Philip  Chiltern+Beaconsfield, Bucks  1984  June  m. Diane Lamb, vol 19, p. 1358, Reg No 684
MCLEOD, Philip  Liverpool, Merseyside 1984 August m. Janice Finnigan, vol 36, p. 376, Reg No 884
MCLEOD, Philip D  Hillingdon, London 1987 July  m. Kay Keeping vol 13. p. 863, Reg No 787
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Joan Pendred is the most likely wife, with the sole Philip for the husband’s name, and from 1910 (she would have been 33 at marriage) to 1927 (16 at marriage) there were these two births:

Surname First name(s) Mother District Vol Page
Births Sep 1919
Pendred Joan Wells Maidstone 2a 1360   
Births Jun 1923
Pendred Joan Taylor Croydon 2a 503
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Births to McLeod-Pendred parents, searched from Q1 of 1942 to Q4 of 1968, when the younger of the above two possible wives would have been 45:

1944, Q2, Janet A McLeod, mother=Pendred, Tonbridge district, vol 2a, p. 2054

No other births can be found, though Philip plus wife may have moved out of England or Wales, she may have died or they may have separated or divorced.  It is not worth looking any further until the marriage certificate arrives, giving his father’s name and deciding whether it was he who married a Joan Pendred or not.
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Aug 2009, TFPL: The marriage certificate between Philip Mcleod and Joan Pendred has arrived.

He was aged 26 (right) and said he was the son of Ernest McLeod (deceased); these two together firmly confirm that this was the right person and Joan P was indeed his wife..  One witness at his wedding was his (half-?) sister Margaret A McLeod, adding to the confirmation.  See the marriage Notes for full details.
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Sept 2009, TFPL: His death certificate has arrived.

He died on 3 May 1988 at Avonmore Barrack Lane, Bognor Regis, W Sussex and was said to have been born on 19th November 1916 (correct) in London.  He was a retired accountant.  The informant was his daughter Janet Ann Chapman who lived at 29 Marshall Avenue, Findon Valley, Worthing, W Sussex.  The causes of death were:
Ia Bronchopneumonia
II Diabetes Mellitus
and was certified by Dr E A Ellis MB and registered at Bognor Sub-district, Chichester, W Sussed on 4th May 1988.
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Sept 2009, TFPL: His birth certificate has arrived.

The additional information on the certificate above that entered for that event was:
Father: Richard Barrington POWYS-LYBBE
Mother: May Mathilde POWYS-LYBBE formerly FRY
Father’s occupation: 1st Lieutenant Army Service Corps (Manager at Motor garage)
Informant: May Powys-Lybbe
Date Registered: 1916.


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On their marriage certificate No 237 for 19 Jul 1943 at Holburn Register office:
He was Philip McLeod, 26 years, bachelor, 1083247 AC2 RAF (Accountant), of 20 Gower Mews Mansions Holburn, father was Ernest McLeod (deceased), father’s profession: accountant.
She was Joan Pendred, 23 years, Spinster, 2012274 LACW, WAAF, of 51 Baltic Road, Tonbridge, kent, father was Walter Ernest Pendred, profession: railway clerk.
The witnesses were Margaret A McLeod [his (half-?) sister] and Walter Eernest Pendred.