Thanks for the replies- all very interesting!
I think that FE Hoppy and spekesoftly have the answer;
The aircraft that I have heard use this tone (quite loud, about half a second duration) have been (I believe) the Red Arrows, a Mirage and, a few days ago, which sparked the question, some Harriers. If the Mirage was a 2seater (possible as it was at an airshow), and the Harriers (operating at night in 'the sticks') were 2seaters practicing field operations, then it all ties in.
Anyone else care to comment?
BTW, Happy Retirement, Heathrow Director- you'll no doubt have directed me (as a pax!) a few times.