I was there at Duxford when the tragic event happened. I'd just landed (R44 helicopter) and the L39 came in 2 aircraft behind me, behind a 172. (He'd been asked to expedite to allow the L39's approach). I'd just stepped out of the helicopter, and the fuel bowser had arrived immediately. I guess it was less than 2 minutes since I'd reached the airfield. As the refueller was starting to pump, we heard a really strange 'whump', then silence. Several other pilots were stood looking around, trying to figure what the noise was, then we spotted the tail of the aircraft just showing over the hedges and knew what had happened. We guessed later that the strange sound was the ejector seat firing, rather than an impact.
From the position of the aircraft, and the hole in the fence & hedging, it was clear that the plane had veered considerably off 06 to the right, and had missed the mound at the end of the runway completely, going through the fence where the ground was pretty level.
Needless to say, that everything stopped immediately while emergency services were rallied. It was confusing for a while, though, as traffic was clearly still moving freely on the northbound M11 for some time after the accident, and the L39 had crossed to the southbound. I'd have thought that everyone would have stopped, and was amazed that people just drove past!
Along with everyone else, I offer my condolences to the family involved. It is a very sad day for aviation when something like this happens, especially involving a superbly maintained aircraft, enthusiastic and experienced pilots, on a good-flying-day, and at a place like Duxford, where many members of the public were there to witness the tragedy.