Complex sentence: In 1951 they began to develop a successor to the Mark 1, which included a floating point unit; the resulting machine, known as Meg, ran its first program in May 1954.
Split into simpler sentences: In 1951 they started on the development of a successor to the Mark 1, which would include a floating point unit. The resulting machine, known as Meg, ran its first program in May 1954.