Want to buy yourself one of the best fitness trackers on the planet? The Fitbit Charge 2 and Fitbit Flex 2 are now both on sale for Amazon Prime Day and each is down to a fantastic price.The Fitbit Charge 2 – a tracker that features in our best fitness bands of 2017 – usually costs £145.99, but now it’s down to £77.39 - which is even better than expected.You...