Ironically I had a telephone call from the cms today, to inform me that my payment would be 'changing' back to whatever pittance it was before, which I already knew as the arsehole stopped claiming benefits, moved half way across the country (not far enough in my opinion) and then started claiming again. In the mean time, I got a letter through telling me he had stopped claiming benefits and would be paying x amount based on whatever HMRC had decided his earnings were, and when I called to query this, was informed of the above, that nope, sorry, back on benefits. What a lovely chap he is.
Anyhoo, I stated again my utter horror that nrp can play this game (obviously signing on/working cash in hand/not declaring correct income, etc rather than contribute to their own flesh and blood - he's not actually allowed to see dc unsupervised, and hasn't for months, thank goodness, but that's another story), and she said that although without solid proof they cannot make this variation at face value, but if you have proof that his outgoings far exceed his supposed incomings for example, expensive car, holidays, high mobile phone bills, mortgage, even down to expensive clothing etc then that can trigger an investigation.
May not help my situation, but I did think of you, whilst I laughed hollowly as she told me she wished there was more she could do. Actually genuinely believed this one as well, she seemed pretty annoyed at how useless the system was as well... Worth a try.