Good point. The use of retroactive legislation to deny people the money that a court ruled they were entitled to was a stain on the British constitution. https://www.theguardian.com/society/2013/mar/15/dwp-law-chan...
Here's the judgement. It's worth reading, because it seems the lack of consultation for a major change was one of the reasons to overturn it.
http://www.bailii.org/cgi-bin/format.cgi?doc=/ew/cases/EWHC/...
(this of course goes on the list of reasons why the government wants to get out of ECHR at any cost, since it's the only constitutional brake in the system)
https://www.gov.uk/government/uploads/system/uploads/attachm...
If it's that far off, there's at least half a dozen editors of major UK papers that'd be rubbing their hands in glee at the opportunity to further demonize benefits claimants by exposing more benefits cheats and/or by exposing flaws in the reporting. That this isn't happening despite both commercial and ideological gain to be had in doing it if they could, is to me a strong indication that the numbers aren't that far off.
Benefit fraud is currently far down the priority list of things that needs to be fixed, and that even goes for cost: The government has pushed things so far that you can look forward to a lot of tax money going to cover legal costs as a result of the huge and growing number of lawsuits the government is losing.
The government is complicit in far worse breaches of legislation related to the benefits system than fraudulent claimants are.