Complex sentence: A sizeable Neolithic settlement is believed to have been in the Heathrow area, as many artefacts have been found in the gravel around what is now the airport, and the Colne Valley.
Split into simpler sentences: A sizeable Neolithic settlement is believed to have been in the Heathrow area. Many artefacts have been found in the gravel around what is now the airport, and the Colne Valley.