I spent a 40 year career working with the police in Hong Kong and London in the fields of Research and Archival Records Management. I set up my own historical research service in 2003 and am a member of AGRA (Association of Genealogists & Researchers in Archives). You will also find me on The National Archives list of recommended independant researchers
From 1996 to 2003 I worked in the Metropolitan Police Archives dealing with family history enquiries. During this time I gained an in-depth knowledge of the Met's archival records held at The National Archives. Joiner's Registers for the period May 1857 - June 1878 have not survived however I have my own database of Joiners which covers much of this time period. Details were obtained from Police Orders and a variety of other sources. Although not a complete record it does go some way towards filling in this gap in the official records.
I would be delighted to assist in the search for your Metropolitan Police ancestor.