His birth index:

First name(s) JOHN G
Last name DAVENPORT
Birth year 1931
Birth quarter 4
Mother's last name TUDOR
District AMERSHAM
County Buckinghamshire
Country England
Volume 3A
Page 1463
Record set England & Wales births 1837-2006

There is only one Davenport-Tudor marriage on the indices from 1900 to 1960 so the marriage certificate was obtained which gave the significant information that the father was Ormus Neville Talbot Davenport.  Interestingly the image of the certificate from the Irish site for the birth of the father gives Ormis as the first name, but the census and all subsequent records have used Ormus as his name.
_________________

There are 7 marriages of John G Davenports on FindMyPast (FreeBMD shows only 4) from 1948, the earliest he might have been married, to 2005 when records cease to be available on the internet:

ID Last name First name Born Died Event Record set  Location   
1 DAVENPORT JOHN G — — 1955 England & Wales marriages 1837-2008  Chapel En Le Frith, Derbyshire
2 DAVENPORT JOHN G — — 1961 British nationals married overseas 1818-2005  Sudan
3 DAVENPORT JOHN G — — 1961-65 British nationals married overseas 1818-2005  Sudan
4 DAVENPORT JOHN G — — 1966 England & Wales marriages 1837-2008  Manchester, Lancashire, England
5 DAVENPORT JOHN G — — 1968 England & Wales marriages 1837-2008  Nuneaton, Warwickshire
6 DAVENPORT JOHN G — — 1981 England & Wales marriages 1837-2008  Watford, Hertfordshire
7 DAVENPORT JOHN G K — — 1952 England & Wales marriages 1837-2008  Caistor, Lincolnshire
8 Davenport John Gordon — — 1955 Derbyshire Registrars Marriage Index  Derbyshire

Of these 2 and 3 are the same couple, as are 1 and 8, though 8 adds that the groom’s second name is Gordon.  Further No 7 has the additional initial of K which is not on the birth registration.

Taking out the name of the record set, this reduces to 5 distinct couples:

1 DAVENPORT JOHN Gordon — — 1955   Chapel En Le Frith, Derbyshire
2 DAVENPORT JOHN G — — 1961  Sudan
3 DAVENPORT JOHN G — — 1966   Manchester, Lancashire, England
4 DAVENPORT JOHN G — — 1968   Nuneaton, Warwickshire
5 DAVENPORT JOHN G — — 1981   Watford, Hertfordshire

The only way to reveal which of these might be his marriage is to purchase all five marriages, a total of £50, though it may be that it is worth negotiating with them to search for the one (if any) that has Ormus (or Ormis, above) Davenport as the name of the groom’s father.
There is a GRO phone number, 0300 123 1837, that can be rung to discuss this or I can e-mail <certificate.services@gro.gsi.gov.uk>.

I need to prepare a table of all the relevant index data to pass on to the GRO.

All 5 were ordered.  As at 6 Oct 2016, 7 days later, the last three did not have Ormus for a father, the second had been rejected and refunded as they could not find overseas marriages and the first had not yet been received.  So the second was reordered over the phone to 0300 123 1837; it looks like the on-line system does not work for overseas events.
_________________

In fact the Sudan marriage was clearly his, Ormus was his father and his second forename is Geoffrey.
________________

He is said by Wikipedia to have been a director of MK Airlines from 1992 to 2003.




________________

There are two children born fairly soon after marriage.  There is also a third born in Liverpool when Jennifer would have been 45.  However there was also a Davenport-Potts marriage in Liverpool in 1975 and I reckon it more likely that the 1975 birth is of the 1975 marriage.
________________

There is no death record, yet, for a John G Davenport nor any probate.
_________________

Ther marriage certificate was issued in The Sudan and conformed to The Non-Mohammedan Marriage Ordinance of 1926; it was half each in Arabic and English.

Serial No: 1320
Certificate No: Kass.P/A/57
Date: 25 September 1960

Details Groom Bride
------- ----- -----
Name: John Geoffrey Dav enport Jennifer Wendy Newton Potts
Age: 28 28
Condition: Bachelor Spinster
Occupation: Bank Manager -
Residence Port Sudan Port Sudan
Nationalty, Rel British, C of E British, C of E
Father: Ormus Neville Talbot Davenport Edgar Valentine Potts
Father Occ: Company Director Dental Surgeon

Married at: Christ church, Port Sudan
Married by: C.I.McN. Smith, Chaplain, Port Sudan

Signed by the Groom and Bride

Signed also by four fairly illegible witnesses.
___________________