Complex sentence: An investor who bought a 1-million-pound apartment in Kensington and Chelsea in December 2014 would have made a speculative gain of £140,000 by December 2015, which far exceeds any income from rent, which is comparatively insignificant.
Split into simpler sentences: An investor who bought a £1 million pound apartment in Kensington and Chelsea, in December 2014 would have made a speculative gain £140,000 by December 2015. This far exceeds any income from rent, which is comparatively insignificant.