Just to update Barnabyp, I have found out the 4 chevrons does indicate a Station Police Sergeant. It was scrapped in 1915 for 3 chevrons with crown on top, so I suspect the photo does show my great uncle Alfred Woodhouse, and must date to before this. Apparantly he was very much involved in the Dr Crippen case in 1910, and kept alot of the original papers, including the original arrests warrents, as 'souvenirs' when he retired. These went to the Scotland Yard Black Museum when his daughter died in 1997, so I may contact them and see if the photo is of any interest, etc.
Thanks again for your help, and the great work you have done for us all getting these records online.