Complex sentence: BHC had been formed by the merger of Saunders-Roe and Vickers Supermarine in 1966, work on the SR-N4 had begun in 1965 and the first trials had taken place in early 1968.
Split into simpler sentences: BHC had been formed by the merger of Saunders-Roe and Vickers Supermarine in 1966. Work on the SR-N4 had begun in 1965 and the first trials had took place in early 1968.