The DWP says 5,000 people were prosecuted last year for benefit fraud and 6,000 'administrative penalties' were imposed.
This is without trying very hard. The aim of benefit fraud policing is to deter offenders and save a bit of money, but mainly to keep a political lid on the issue of benefit fraud. It is not being ignored, because that would bring protests.
But equally it is not being actively addressed. If the government really wanted to attack benefit fraud, it would blitz an area and see how much money could be saved, starting by checking databases and getting leads from local government employees. Publicising the blitz beforehand would probably result in some benefit claims being, um, discontinued.
But it's not about saving money, it's about keeping the political lid on the issue. So widespread, understated benefit fraud will continue.