Complex sentence: The Ariel Hotel built in 1961, and designed by Russell Diplock & Associates was "Britain's first significant airport hotel", built at what was then London Airport, and which is now known as London Heathrow Airport.
Split into simpler sentences: The Ariel Hotel built in 1961, and designed by Russell Diplock & Associates was "Britain's first significant airport hotel". It was built at what was then London Airport, but is now known as London Heathrow Airport.