When were women first recruited by the Metropolitan Police in London
and when were women first admitted to the plainclothes staff of the
CID?

Hello bow33

The Metropolitan Police first directly employed women for policing
duties in 1918.  The first female CID officer was Lilian Wiles, who
was appointed in 1922/23.

According to information on the web site of the London Metropolitan
Police, although the Women Police Service had been founded in 1914,
and they were permitted by the Police Commissioner to patrol the
streets, mainly to carry out rescue work among prostitutes, they were
not at that time formally used for duties by the Metropolitan Police. 
In fact, the first official appointment of a policewoman occurred not
in London, but in Grantham in 1915.

The Metropolitan Police favoured a rival body, the Women's Special
Police Patrols.  In November 1918, the Commissioner, who was then Sir
Nevil Macready, appointed Mrs Stanley as Superintendent of the
Metropolitan Women Police Patrols, and at the same time 25 women from
the Women's Special Police Patrols were taken into direct employment
by Scotland Yard and assigned to policing duties. However, these women
were not sworn in and did not have special powers of arrest. Women
police received limited powers of arrest and were fully attested in
the period 1923-1930. A4 Branch (Women Police) was established in
1930, but it was not until 1973 that women were integrated into the
main force of the Metropolitan Police.
http://www.met.police.uk/history/women_police.htm (Metropolitan Police web site)

According to the research of Harry Wynne, Lilian Wiles was the first
woman to be admitted to the CID, although he only places the event as
being in 1922/23. Incidentally, the report of a lecture by Harry Wynne
given at the URL below also has details of earlier ?policing?
activities by women, for example he found a reference from 1899 to:
?the London female police, apparently a group of women in Covent
Garden who became, in effect, "traffic officers", trying to control
the chaotic traffic around Covent Garden. The carters, naturally,
objected to this and would hurl potatoes and turnips at the women.?
http://www.ndfhs.fsnet.co.uk/Reports/gh_2002_05.html (Report by
Margaret Stafford of a lecture given by Harry Wynne at the
Northumberland & Durham Family History Society Gateshead Branch
Meeting 2002)

Good answer.  Told me what I needed to know.