Two Nimrods and a P3 observing the passage of Russian Naval units well out to the west of the UK. The captain of the other Nimrod, a Wing Commander, had a group of journalists on board and he suggested that we formate on the P3 so that his passengers could get some photographs of the two of us with the Russians in the background. I knew that the captain with whom I was flying was a very capable pilot but I recognised a potentially dangerous situation when I saw one, unplanned, unbriefed, unpracticed formation of two different aircraft types. So I concentrated my attention inside the flight deck while both the pilots were looking outside. We joined on the P3's starboard side partially astern and slowed to his patrol speed. The pair of us flew down one side of the Russians, the P3 between us and the ships, and started a 180* port turn to fly past on the other side. About ¾ round the turn the captain called “Speed check”. The P3 had lost a little speed in the turn and the Nimrod was beginning to feel a bit sloppy. No reply from the co-pilot. The captain called “Speed check” a second time and got no reply so I slid my seat forward and turned so that I could see an A.S.I. Three things then happened almost simultaneously. The captain looked down, so I looked up, and the P3 rolled out of his turn. I remember changing my shout from “Look out” to “Look up” just before the tail of the P3 passed above our flight deck at a distance of less than 10 feet. We broke downwards and sat quiet for quite some time. Nothing was said because there was nothing to be said.