RK 787 would be a good guess as this was the prototype as signified by the circled P.

As were ‘791, and ‘794 (which became the tug proto) which also wore ‘P’s

Some nice Sturgeon pics can be seen on Richard Ridings’ page,

http://www.google.co.uk/imgres?imgurl=http://rodb.gofreeserve.com/Av4/Thunbs/Thmb_Short%2520Sturgeon%25202.jpg&imgrefurl=http://rodb.gofreeserve.com/Av4/RBAvpic4.htm&usg=__m8RoY_r_rVKNXfiMbkbzKUh5t-k=&h=65&w=98&sz=2&hl=en&start=9&zoom=1&um=1&itbs=1&tbnid=yOYU8t9f3L7e9M:&tbnh=54&tbnw=81&prev=/images%3Fq%3DSTURGEON%2BRK791%26um%3D1%26hl%3Den%26sa%3DN%26tbs%3Disch:1

and the SB.3 here,
http://www.google.co.uk/imgres?imgurl=http://www.fliegerweb.com/inhalte/Geschichte/Flugzeuge/ShortSB3/ShortSB3_620xx_2.jpg&imgrefurl=http://www.fliegerweb.com/geschichte/flugzeuge/lexikon.php%3Fshow%3Dlexikon-611&usg=__wfCxsusCl1beQK0LwB5AzR_19gQ=&h=334&w=620&sz=19&hl=en&start=19&zoom=1&um=1&itbs=1&tbnid=d9d6Jsg9LkDqjM:&tbnh=73&tbnw=136&prev=/images%3Fq%3DSTURGEON%2BRK794%26um%3D1%26hl%3Den%26sa%3DG%26tbs%3Disch:1