He is mentioned in one of Philip L P L codicils to his 1870 will as the husband of Mabel.
The BMD marriage index, which does not give the spouse for either, has two entries on the same day and parish, one for Mabel and one for Herbert.  So this corroborates the codicil.  One day I must get the marriage certificate!
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July 2003: The marriage cert est arrivée!
Married on 27 June 1885 at St Paul's Chapel of Ease of the parish of St John Battersea, Surrey; register number 63.
Him: Herbert Bowker, aged 33, Bachelor, an engineer, of 5 Wilson St, Father = Henry Francis Bowker, Gent.
Her: Mabel Lybbe, aged 19, spinster, no occ., of 10 Munster Terrace, father = Philip Lybbe Powys Lybbe, Gent.
Witnesses: Charles Summerfield Wildman, Rose Edith Wildman, Florence Emily Wildman
Celebrant: A L Manby.
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No sign of any death on FreeBMD up to 1925 but we know he was a mining engineer and probably lived abroad.
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There were several Bowkers on passenger ship going to all parts of the globe.  All we know is that young Herbert gave his father as a Mining Engineer of Transvaal in 1914.  There is a pair of Bowkers going to East London, South Africe on 26th June 1911 on the Balmoral Castle of the Union-Castle Mail Steamship Co Ltd, departing from Southampton; this might have been them or it might have been someone completely different.  (FindMyPast Passenger Lists.)
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The 1891 census is a bit odd:

Name Relation Condition Sex Age Birth Year  Occupation  Disability  Where Born

Bowker, Herbert Head Married M 39 1952 Electrician  London
BOWKER, Annie M Wife Married F 30 1861 [blank]
BOWKER, Claude H Son M 3 1888   Brighton, Sussex
BOWKER, Violet G Daughter F 0 (8M) 1891  Chelmsford, Essex
CORNISH, James H Lodger M 22 1869 Electrician  Bridgwater, Somersetshire
STAINES, Clara Servant F 14 1877 General Servant Domestic  Dunmow, Essex

RG number: RG12 Piece: 1386 Folio: 63 Page: 13    

Registration District: Chelmsford Sub District: Chelmsford
EnumerationDistrict: 11   Ecclesiastical Parish: St Johns Moulsham

Civil Parish: Chelmsford Municipal Borough: Chelmsford
Address: 2, Joy Villas, Hamlet Road, Chelmsford, Moulsham
County: Essex

The problems:
1. In the transcript, he had been left out altogether, so I’ve put in his correct details from the original enumerator’s form.
2. On the original form her birth place is clearly blank, so I have omitted the ‘London’ bit on the transcript.
3. His wife’s name and age is given as Annie M, aged 30, so born in 1861, clearly at variance with Mabel Lybbe’s birth date.

Otherwise I think that this record clearly includes Claude Herbert and at least one of his parents.
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Violet had her birth record as follows:

Surname First name(s) District Vol  Page
Births Sep 1890
Bowker Violet Gladys Chelmsford 4a 4_3 (probably 443)

It looks sensible to obtain her birth certificate.

She is not on the 1901 or 1911 census and there are no marriages or deaths on record for her.

Jan 2012, TFPL: At last got round to getting her birth certificate and, as I thought, her mother was recorded as “Annie Bowker formerly Daniels”.
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Jan 2012, TFPL: On enlistment for the Australian Imperial Force in 1914, his son Claude Herbert Bowker gave this info for his father:

Next of kin Herbert Bowker, Nigil Gold Mining Coy near Heidelburg, Transvaal, South Africa.

There is quite a bit of information on this company on the internet.  From a Journal in 1919, the mine was spelled “Nigel” and the locality was “Heidelberg”.
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Their marriage, from FreeBMD:

Surname   First name(s)      District   Vol  Page
Marriages Jun 1885   (>99%)
Bowker Herbert Wandsworth 1d 939
Lybbe Mabel    Wandsworth 1d 939
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