I was chatting with a good friend a few months back about this - he's a fraud investigator for an insurance company and was talking about the rise in this type of claim. He was saying that about 50% of his team's time is spent dealing with this, whereas 5 years ago it was rarely heard of (most of his time was spent on "stolen" cars torched, either en mass at "professional" premises or in the supermarket car park at night, with s call to the police in the morning ("woke up and my car was gone...")).
His main alert is when the collision involves a hire car with 3/4 occupants making a personal injury claim, closely followed by ratty car in the same circumstances...