Hello Elsbeth
What a wonderfully interesting life yourself & your late husband have led !
I've been doing some digging - I hope this will not swamp you! I'm trying to put it in a reasonable order

From what I can gather...there is a generation missing in your outline.....
The 2 photos you have of the WW1 soldiers brothers Carel Archie and Ferdinand DE WITT, were certainley born and died in England, and undeniably are sons of Carel Blad DE WITT (b Netherlands 1855) and Helene Jane GARDINER (b Scotland 1864) who were married in 1886 in Lancashire, and who also died in England.
You have said, Carel Archie b 1892 and Ferdinand b 1888 are cousins of Ferdinand (Fen) DIEPHUIS born 1881 Groningen - do you believe this relationship to be 1st cousins:
There is a Carel Blad De WITT on the IGI born 1855 Groningen whose parents are listed as Ferdinand DE WEEMA and
Elisabeth WEEMA.
This is the ful list of children on the IGI, born in Groningen to Ferdinand de WITT and Elisabeth WEEMAWillem Hendrik DE WITT b 8 Apr 1839 Burgerlijke Stand, Groningen, Groningen, chr 10 Nov 1839
Anna Clara DE WITT 21 NOV 1840 Burgerlijke Stand, Groningen, Groningen
Elisabeth DE WITT 03 NOV 1846 Groningen (submitted record)
Carel Blad DE WITT - born 10 Oct 1855 Groningen, Groningen, Netherlands
Lambertus de WITT b 28 Sep 1858 Groningen, Groningen, Netherlands
( Note: These births are all extracted, except for the daughter Elisabeth. "Extracted" record - meaning it's a transcript from actual parish or church or civil records as opposed to submitted record by family members or LDS sealing records which can be unreliable).Also on the IGI (Extracted):Ferdinand de WITT age 38, (born 28 Sep 1799 Groningen son of Willem de WITT and Anna EIFELD)
married 28 march 1838 Burgerlijke Stand, Groningen, to
Elisabeth de WEEMA age 18 (b3 Jan 1820 Groningen dau of Hindrik WEEMA and Elizabeth Ennes WIERSEMA)
It's also possible Ferdinand was married before to Zusanna van BELKUM in 1828, Burgerlijke Stand, Groningen - which is from another extracted IGI record. This couple had a son Willem de WITT in 1829 Groningen.
So I get a generation summary as follows:1. Ferdinand de WITT (1888 Eng-1949 Eng) Carel Archibald (1891 Eng-1929 Eng), WW1 veterans & sons of:
2. Carel Blad de WITT (1855 Neth'd-1928 Eng) and wife Helene Jane GARDINER (1864 Sct-1933 Eng) [married 1887, England]; Carel son of:
3. Ferdinand de WITT (1799 - 1885) and Elisabeth WEEMA (WIEMA) (1820 - ~?) [married 1838]; Ferdinand son of
4. Willem de WITT and Anna EIFELD
HERE is a link to the Groningen Archives websitehttp://www.groningerarchieven.nl/content.php?hoofd_id=3&sub_id=45&subsub_id=49&hofman_id=35It has a photo of Ferdinand de WITT, encaptioned to say:
born28-09-1799 to died 28-10-1885, lid van de gemeenteraad
http://www.archieven.nl/pls/m/zk2.obj_start?p_id=39926235&p_vast=5This certainley seems to be a photo of the man who married Elisabeth WEEMA - his birth date matches that stated on the IGI extracted record for his marriage to her. I doubt very much whether Ferdinand was ever in England, at least he did not die there?
Further, I also found on the Archives, the possible marriage of his daughter Anna Clara:
http://www.archieven.nl/pls/m/zk2.inv?p_q=43084221 (on that page, click "details")
It says, I think that an Anna Clara de WITT married in 1869 to Franciscus LIEFTINCK (who had been widowed by his first wife Anna POSTUMA).
So if that's the right Anna Clara de WITT, that's taking me further away from finding who of the De WITT line married Dr DEPHUIS - Maybe there were other de WITT daughters, who I can't see on the IGI?
I wonder:
Is this Dr DIEPHUIS, Fens father?
http://en.wikipedia.org/wiki/Gerhard_Diephuis
Gerhard Diephuis (1817-1892) was a Dutch jurist.
After studies in Groningen, he was a judge with the Arrondissementsrechtsbank in Winschoten and later at the Gerechtshof in Groningen. In 1859, he took up a teaching post at the university of Groningen.Diephuis was a leading proponent of legism and of the primacy of legislation. Influenced by French teachings, he wrote numerous textbooks on Dutch civil law. He was also instrumental in further diminishing the influence of Roman and customary law in favour of codified legislationPhoto of him here:
http://www.archieven.nl/pls/m/zk2.obj_start?p_id=39926098&p_vast=5OR: There is also this Groningen - Dr. Johann Heinrich DIEPHUIS?
http://www.archieven.nl/pls/m/zk2.obj_start?p_id=39926101&p_vast=5http://www.archieven.nl/pls/m/zk2.obj_start?p_id=39926100&p_vast=5Cheers
AMBLY