JSA Sanction!

Hello, I have worked pretty much all my life but a few years back we got in trouble with HMRC due to us not informing them that i had moved in with my partner. I have put this down to having a new baby and my partner suffering for Post natal and also having C section ment I had to pick up the slack. It was something that got forgotten about, Bad idea as they now say we owe them £8000. We have accepted that we will have to pay this at some point. My Issue with JSA is that I was working at a new ASDA store and when i went to the interview I was told overtime wouldnt be a problem and there would be loads of it so that ment I was able to claim working tax credits as on average I would be working more than 24 hours a week which is what is required to be entitled to it, after 3 or 4 months of working there it came to light that there would be no overtime as the store wasn't doing as well as hoped, because of this i ended up leaving my job as due to the huge debt with have with HMRC we didnt want to end up owing them another £8000 or something and If I had stayed working there we would have been getting in £260 debt every month, Being on JSA seemed like my only option at least until i could find a full time job, Since i claimed though I have now been put under sanction until June/July and receive no JSA. Is that even legal? how can they expect a family of 4 to survive on £120pw which is from child benefit and child tax, I am currently trying to set myself up as self employed however I am stuck as if i go to claim working tax credit again as self employed they will stop child tax credits until its all sorted which would mean we would have to live off 20-30£ per week. Has anyone else had similar experiences? I would love to hear peoples points of view on this matter

Thanks Rant over