A friend on my master's course was a Chevening scholar and she had to go home at the end - she said they were "cracking down" on people staying, but I'm afraid I don't know much more than that. But one of her friends got an offer for a funded PhD and then wasn't able to accept it, so I think if you did get this scholarship, you might have to accept it means two years back in your home country. A two-year wait before a PhD isn't so bad though - perhaps try to see it as an opportunity to get some relevant experience and publish some papers? Then you'll be really competitive by the time you can apply again!