I've had a lovely time doing this for Ashleigh! Here is a timeline I've produced with the results of the research I've done so far.
NB There’s another Charles French who worked for the royal family at Sandringham as a gamekeeper but he was Charles George French and research shows he’s not related to Ashleigh.
Charles Nelson French
14 June 1880 Charles was born in Southminster Essex (Ashleigh has some family history data so I didn’t research the family)
30 June 1900 joins the royal staff as a groom based in London earning 21/- per week. Source: Royal Mews Establishment Lists 1889 - 1908 and 1860 - 1919
March 1901 living at the Royal Mews next to Buckingham Palace working as a groom and stable helper. Source: Census 1901
1906 - 1910 earning 23/- per week as a groom based in London (Buckingham Palace). Source: Master of the Horse's Department Establishment Books 1906 - 1910
(6 May 1910 King Edward VII died and was succeeded by King George V)
28 August 1910 Charles was put in livery. Source: Royal Mews Establishment List 1914
October 1910 Charles is put on the payroll at Windsor. Source: Wages Book for Weekly Helpers employed in the Royal Mews at Windsor Castle 1904 - 1935
25 November 1910 Charles moved to Windsor. Source: Master of the Horse's Department Establishment Book 1920
1911 - 1912 earning 23/- per week as a groom based at Windsor & now wears a scarlet and plain livery. Source: Royal Mews Establishment Lists 1911 and 1912
November 1913 Charles married Harriet May Dye in Dersingham, Norfolk. His parish was named as West Newton (within Sandringham estate). Source: Norfolk banns & marriages. This indicates that Charles travelled with the family to Sandringham when they visited.
1914 earning 27/- per week as a groom based at Windsor, wearing the scarlet and plain livery and ‘living in'. Source: Master of the Horse's Department Establishment Book 1914 and Royal Mews Establishment List 1914
1915 Charles attested for the army under Lord Derby’s scheme. Source: Wages Book for Weekly Helpers employed in the Royal Mews at Windsor Castle 1904 - 1935
1916 and 1917 earning 25/- per week as a groom based at Windsor plus 2s lodging allowing for 'living out'. He wears a scarlet & plain livery. Sources: Master of the Horse's Department Establishment Books 1916 and 1917, Royal Mews Establishment Lists 1916 and 1917
1919 earning 25/- per week as a groom based at Windsor plus 2s lodging allowance for 'living out', plus 29/- ‘war bonus’ (they all get a war bonus) - wears a scarlet & plain livery & is living out. Source: Royal Mews Establishment Lists 1919
1920 earning 27/- per week as a groom based at Windsor plus 32/- ‘war bonus’ (they all get a war bonus) - wears a scarlet & plain livery & is 'living in'. Source: Master of the Horse's Department Establishment Books 1920, Royal Mews Establishment Lists 1920
1923 earning 27/- per week as a groom based at Windsor plus 18/2 ‘war bonus’ (they all get a war bonus) - wears a scarlet & plain livery & is 'living in'. Source: Royal Mews Establishment Lists 1923
1924 Charles paid up to 16th August at Windsor then his wages entry is blank. Source: Wages Book for Weekly Helpers employed in the Royal Mews at Windsor Castle 1904 - 1935. (My assumption is that he then came under a different department for payment.)
1921 - 1935 Charles and his wife Harriet (May) are on the Electoral Roll at 9, Red Buildings, Royal Mews, Windsor Castle from 1921 - 1925 and at 6, Royal Mews, Windsor Castle from 1926 - 1935. Source: Electoral Rolls for Berkshire
1939 Charles and his wife are at Hall Stables, Sandringham. Charles is recorded as a retired groom. Source: 1939 Register
(We’ve seen photos of Charles with the royal family during the war years so my assumption is that he came out of retirement as part of the war effort).
April 1950 Charles was buried in the churchyard of St Mary Magdalene, Sandringham - address given as Royal Stables, Sandringham. Source: Norfolk Parish Registers.
Here is his grave:
https://www.findagrave.com/memorial/202353876/charles-n-french
Sources were mainly found on ‘Find My Past’ and one of the electoral rolls on ‘Ancestry.co.uk’