I got it Chris!
Very distinctive sound. I recall mentioning on here a few years ago that walking off Riddlesdown in the last glimmerings of light I'd hear a distinctive noise and seen a bright red anti collision beacon several miles away heading towards Biggin. Could only have been a Dove.
There was a Heron which arrived in the Duxford overhead several years back... beautifully quiet were those four Gipsies... I believe it's currently down but not necessarily out. Edit: has a current CofA, is it flying? Hope so...