However recently I put in the cm calculator what he told me he gets from renting out the rooms
He's not very bright, your ex, is he?
and I was surprised to see how much he should be paying as I didn’t realise it would be so much. This has prompted me to want to report it now but as it’s probably all cash in hand I probably won’t get anywhere.
Cash in hand is easy to prove - he has subtenants living in his home!
Speaking to a family member about it they told me to just forget about it and said it just seems bitter.
Family member is a tool to have said that to you.
Firstly - who wouldn;t be bitter about it, secondly, it;s not "bitter" to want your child to have decent provision made for them by both parents.
So wibu to report him to hmrc? Would it be worth it?
Don't know enough about the legal specifics, but it's worth doing something.
Think I'd start with the HA first though. No need for subterfuge - just ring up, explain your DC's father is refusing to pay for his DC & is hiding money from the CMS, & you need evidence that he is subletting.
That will at least alert them - & you - as to whether he has declared the subletting to the HA formally.
If he has - bingo, there's your proof.
If he has not - bingo, the HA are going to be investigating, & you will have your proof.
The main problem I can see is that the HA may not wish to discuss details of a tenant with you. That's why I'd make it a phone call in the first instance - you'll be able to convey the unfairness to your child, & may get a sympathetic ear & a few hints on how to tackle it.
Be wary of an HMRC or DWP report though. When they cut off his benefit, he really will have no money to send his child. Unless you are more concerned with revenge/just desserts than cash. For which I cannot blame you!
Good luck.