Hi,
After a little advice, don't know where else to turn and don't want to get in trouble.
Split with ex last year, we have a child together. We lived together and obviously claimed the working tax credit we were entitled to, jointly. The relationship broke down, luckily it was mutual, no hard feelings, just grew apart due to Constantly bickering and just making eachother unhappy. He bought a house when we split, I moved to my Mum's.
Anyway, he earns a fairly good wage, I on the other hand, don't. I'm self employed, and can only work two days per week whilst little one's in nursery, can't afford any more days. Obviously this will change once my child is school age, but for now I struggle.
I can't fault ex in terms of maintenance, he pays what he can, I couldn't ask him to pay more than he currently is, also he has a bedroom full of clothes and toys for our child, and always goes half on clothes/toys for DD etc... he's a good Dad and a good guy in general, but we will never reconcile.
Anyway, my Mum has had enough of me. She's difficult to live with, extremely moody and just wants her own space, which I can understand, and to be honest so do I. She wants me out asap 
I don't know what to do, I don't have the money to get a place of my own straight away, i'd need furniture/deposit etc.. it's not do-able at present, i'd need a few months to save.
I've spoken with ex, to ask if he can help towards deposit etc.. but because he has not long bought his house, he is a bit strapped at the minute, so he proposed I move in to his new place with DD. His cousin is a boss in the marines, is barely ever home and has a flat just sitting doing nothing, he has agreed ex can move in there for as long as is necessary and he only wants him to pay the council tax, nothing else! Ex has said he'll keep paying all the bills and the mortgage, I don't have to pay anything whilst i'm there, I can concentrate on saving for my own place.
I've worked out the sums, and it looks like i'll need to take him up on his extremely generous offer for about 4 months, hopefully a bit less if work picks up.
Anyway, I know when I move, i'll need to inform tax credits/HMRC as my tax credits renewal is due as well as my NI. I'll be letting them know it's only temporary. I'm worried sick about this being illegal as it's still ex's home and bills are in his name? He's said he'll inform HMRC of his new temporary address so we won't be linked that way through HMRC.
Is this allowed? We can't do a joint claim as we aren't together and that's illegal, plus i'd lose some working tax credits that I need to live off in order to save for a new place. I'm scared they'll find out it's ex's property and assume we've reconciled and stop my tax credits and prosecute me.
If anyone could tell me what they believe the best course of action would be before I actually go ahead with moving in, i'd be very grateful. Bare in mind, it'll be for about 4 months, at an absolute push in worst case scenario, 6 months.