The HMRC are powerless to stop anything but the most obvious cases of tax evasion at the moment. This government is making 10,000 of their already overworked staff redundant by the next election. There simply isn't enough staff to sift through the more complex cases. Reducing staff seems like madness at a time when this country desperately needs more tax revenue to plug the deficit.
I've stated this before on forums but no one seems to get it.
Many on here state we have too many civil servants but they haven't yet said who they want to replace them and chase people who evade tax and cheat benefits. It's open season for fraudsters at the moment with the amounts of cuts to civil services.