Belay at the top of P1 of britomartis. Constructed of 1 in-situ peg, 1 in-situ nut and 2 additional nuts. All 3 nuts were behind a large flake that appeared solid at the time of belay construction. Climbing as a 3. When 2 seconds arrived the flake flexed with them hanging on it and all 3 nuts popped out. Belay was well equalised and held on the peg (heart in mouth). We all calmly unweighted the belay and added 2 more nuts and a cam in to the mix. The cam and 1 nut was good, the other nut behind another creaking flake. Equalised it all. Could have been an incredibly serious incident if it were not for the old peg.