Time to draw this mystery to a conclusion. It is the 1916 Grahame-White Twin Airscrew Experimental A8964. It has also been given the designation Grahame-White M.1 but there is no evidence of that being contemporary. Very little is known about it. Some sources say that it had a pair of contra-rotating propellers (that suggestion does not rest easily with what can be seen in the photograph). Others say that it had two four bladed airscrews, bolted ‘front to back’, which both rotated in the same direction. No-one seems to know what powered it although there are some suggestions that, if the propellers were contra-rotating, there were two engines within the cowlings. As mentioned already, whilst a government contract for it was placed, it was not delivered to the RFC (sorry for the earlier inappropriate reference to the RAF). What became of it does not seem to be recorded, leaving this oddball as a footnote in aviation history. Open house, please.
P.s. the sole photograph of it is to be found on p.41 of ‘Hendon Aerodrome – A History’ (David Oliver, Airlife, 1994).