Sea Horse in the Sky — Edmund Cooper

At first he though it was all part of some crazy nightmare. But it wasn't. Russell Grahame, M.P. was one of a handful of passengers flying from Stockholm to London. One moment flying peacefully in the sky, the next lying in an un-Earthly green coffin. Grahame was the first to emerge from this strange resting place. But for him, as well as for the others, it had been only the ecliptical experience. Soon all were to find themselves lost in a bizarre world of Mediaeval knights, Stone Age warriors and gremlins, caught unalterably in the weirdest cocoon of Time.