Complex sentence: The area beyond West Ruislip fell within the designated Metropolitan Green Belt, and as the intended housing developments were no longer allowed the extension was cut back to West Ruislip; it opened in stages in 1947 and 1948.
Split into simpler sentences: The area beyond West Ruislip was within the designated Metropolitan Green Belt and the intended developments were no longer allowed. Consequently, the extension was cut back to West Ruislip, and opened in stages in 1947 and 1948.