My experience with the CMS. Have 2 DC with ex.
He used to pay, not always on time and not a lot. But he did. Then he met his now wife.
I received a letter from CMS stating that as he now had responsibility for 2 more DC (not his) my maintenance was reduced.
Then they (ex and wife) decided that I didn't need actual money one month. A bag of her sons cast offs were a suitable replacement. Strangely my DC can't eat clothes, and his old stretched out of shape/faded summer clothes weren't much help in November.
Then he accidentally sent me a message meant for her saying how much he hated living in his flat and not with her. I assume this was so she could still claim as a single parent. So I forwarded this info to CMS. After all, the reduction for her dc is no longer applicable if he's moved out. They said there was no evidence that I hadn't faked the message and I needed to prove he lived elsewhere. His phone number (which they had on record) was shown in the screenshots.
He then quit his job as apparently she couldn't cope with 2 DC on her own, they had 3 more and CMS decided he couldn't afford to pay anything but would persue him for the debts.
They told me over the years that they can't take his driving license away as he needs it to get a job. I don't have one, but UC have never accepted that as a reason I can't work. Seeing as there's a thing called public transport!
Last I heard they'd decided to wipe the debts as he can't pay them and DC are adults now. Which is so kind of them as I'm being threatened by bailiffs for debts I've accrued over the years of not having enough money. Maybe if they actually did their job I could sort that out. Wonder if the bailiffs will wipe my debt on the basis that I can't afford it 
CMS are a fucking disgrace.