From his birth certificate, transcript by me:

CERTIFIED COPY OF AN ENTRY OF BIRTH
Registration district:  Kingsclere and Whitchurch
1942 Birth in the Sub-district of Kingsclere and Whitchurch in the County of Southampton

Cert NO:          314
When/where born:  23rd April 1942 Ashe Park Ashe Overton RD
Name:             David Spencer
Sex:              Boy
Father name:      Patrick Spencer Boyle
Mother name:      Rita Sylva Boyle formerly Berens
Father job:       Lieutenant R.N.V.R. (Parliamentary Candidate)
Informant:        R S Boyle mother Ashe Park, Nr Basingstoke Hants.
                    - as per declatayion dated 26-8-42
Date registered:  27th May 1942
Registrar:        J B Haulton

Given at the GENERAL REGISTER OFFICE under the seal of the said Office, the 12th day of May 2021
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I wonder what the declaration of 26th May 1942, the previous day to the registration, was?
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There are three ‘David S Boyle’ marriages from 1962 when he would have been 20:

Year       His Age   Bride’s name         Marriage district
1980, Q2    38       Malanie G Foster     East Dereham, Norfolk
1991, Jun   49       Deborah M Tiplady    Burnley &N Pendle, Lancs
2003, Mar   61       Carin Bradley        Kendal, Westmorland

Sort of buying certificates I cannot tell if any of these are him.

He may have emigrated of course.
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FindMYPast shows no death records for any ‘David S Boyle’.
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David Spencer was indeed very alive in 2003 when he was an executor for his mother’s will.  I summarised the additional information in this will as:

“It sounds as if her son David had two families, one that produced Robert and a second family that produced the much younger Fenella and Jamie.  This may lead to identifying their mothers and the marriages of David Spencer.”

There is a Robert A R Boyle born in 1963 that sounds like it could be Robert Algernon Radcliffe Boyle.  His mother’s maiden name was Harris but no marriage is to be found; possibly as David Spencer was a bare 20 or 21 at the time they decided not to get married.

Now for Fenelia and Jamie.  A Fenella Maria Boyle was born in uWestminster in Q1 of 1982 and her mother’s maiden name was Foster.  A Jamie Patrick W Boyle was born in Q4 of 1983 also in Westminster and his mother’s maiden name was Foster as well.

So their mother is the first marriage listed above.  Sounds like some good sleuthing!
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With a marriage established between him and a Foster, there are 7 births between 1979 and 1999 with the Boyle surname and a mother’s maiden name of Foster: 2 in Blackpool, 2 in Huntingdon, one in Bury and 2 in Westminster.  The two named in their grandmother’s will, born in 1982 and 1983, are the 2 Westminster births and I am going to assume that if there had been any more children to David and Melanie they would have been added to their grandmother’s 2002 will.  So David and Melanie just had the two children: Fenella and Jamie.
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He was on the Electoral Rolls at 52, Woodsford Square, London, London, W14 8DS from 2002-19

He was on the Electoral Rolls at The Dower House, Chipping Norton, Chipping Norton, Oxfordshire, OX7 5TL from 2002-2015 and this just happens to be the address he gave when signing up to execute his mother’s will in 2003.
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Their marriage index:

First name(s)       David S
Last name           Boyle
Marriage quarter    2
Marriage year       1980
MarriageFinder      David S Boyle married one of these people
                    Melanie G Foster
Spouse's last name  Foster
District            East Dereham
County              Norfolk
Country             England
Volume              10
Page number         1448
Record set          England & Wales Marriages 1837-2005
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