Complex sentence: The FX4 London taxi was the successor to the Austin FX3, which was produced between 1948 and 1958, which in its day was the most widely used taxi in London.
Split into simpler sentences: The FX4 London taxi was the successor to the Austin FX3, which was produced between 1948 and 1958. In its day the FX3 was the most widely used taxi in London.