Owners Standard Steam Fishing Co.Ltd.Grimsby.
Funnel livery all black funnel carrying a white letter S.
Saon scrapped August 1958 so date earlier than that but Steve Farrow may be able to date the photograph more precisely by the tug!
Great atmospheric photograph you've posted shipmate 17.
The fish dock coal-burning tugs were replaced by new diesel tugs about 1964. These included the Sir Alfred Bannister, launched 26.9.1963, William Grant, 22.3.1963. Herbert Crampin, 11.1.1966 and the Jack Croft Baker.