Sandringham 1 you stated
I am going by the gap between the fuselage and lower wing, the scalloped trailing edge, the just visible 45' strut at the far left of the picture that went from the short lower wing tip to the longer upper wing tip, and the bit of strut that is mostly obscured by the persons right hand/arm, it went out to the forward tip of the landing skid.The cowling is in a style fitted to several aircraft types using rotary engines, the Sopwith Tabloid being one and some Caudrons were build in the UK with Gnomes so a cowl of that style would be an option.
Whilst I agree with your comments regarding the similarity to the Caudron G3's wing, scalloped trailing edge etc i have yet to find a photograph of a RFC or RNAS Caudron G3 where the fuselage is mounted above the lower wing. In the Windsock Datafile No 94 on the Caudron G3 there is a photograph on page 15 of a Danish G3 and the caption states "Denmark's only G3 was
unusual in that it is a G3 with the nacelle mounted above the lower wing..." .I have searched J M Bruce book "The Aeroplanes of the Royal Flying Corps" and Own Thetford book "British Naval Aircraft since 1912.and all photographs show the lower wing abuts against the bottom of the fuselage nacelle.The only photograph of a fuselage nacelle above the lower wing is that of Caudron No 311 obtained by the War Office in may 1913. J M Bruce states that this aircraft until 1st April when following an adverse report by the Squadron it was condemned.was used by No 4 Squadron The aircraft in the photograph could of course possibly be Caudron No 311. although the photograph published of the aircraft after acceptance by Scarborough has no cowling fitted.
Just a thought could it possibly a RNAS Coudron G.IV,, a twin engined derivative of the G.3, of which i understand they had 55 delivered?