This may be a bit of a ramble, but I'll try and give the full picture.
Ex-P and I split up around 4 months ago. Since then he's not stuck to any of the arrangements we made regarding access to the children, or money he's supposed to give me.
Our first agreement was that he would have both of the children overnight once a week, either a Friday or Saturday night. We have a 2 year old and an 8 month old. He's had them both overnight approximately 2 times. More than once he's been and picked them both up, or I've taken them to him only for him to bring the baby home a few hours later. His excuses have ranged from she's crying and he doesn't know what to do, to he needs to take our 2 year old to get his hair cut the next day and he can't take the baby with him. Once he bought her back to me at 3am, dropped her off at my friends house where I was. I've had a completely shitty couple of weeks, my Grandad, who I was extremely close to, died last Tuesday while on holiday. I've had the worst 2 weeks of my life, trying to cope with that and tonight I had plans with friends to try and take my mind of it, and just to vent for a while, but he has once again let me down and is now saying he can't have the children for me because he has to meet his football team manager in the pub to discuss fines 
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Our second agreement was about money. We said he'd give me £20 a week, not a huge amount but he doesn't earn much and £20 would cover nappies/milk for the baby and a couple of things for our son. He's given me this £20 once, in the four months we have been apart. He always has an excuse. I'm living off benefits now, and while I do get enough to get by that £20 would be a huge help, but instead he chooses to go out drinking every weekend, which is where his money goes.
He also owes me money, he's put holes in 3 of the doors in the house through punching them, and one wall. I paid for 2 new doors because the landlord was coming to inspect the house, and he said he'd give me the money back. I have, of course, not had this money, and he has since put a hole in one of the new doors. I also lent him some money when we first split up and he's not given it back. I should probably add we're only young, I'm 20 and he's 23. Please save me the lectures about doing things too young, I know, but the kids are here now and of course I wouldn't change them for the world.
We had also run up some rent arrears from a few months ago when ex-p was off sick from work. When we split up he said he would take some money down to the estate agents each week to pay off the arrears. He's not paid a penny, he says because he knows once the arrears are paid off I will move away with the kids. I do want to move out of this house, but not miles away, and definitely not so far away that he wouldn't be able to see them. So I am now trying to save as much money as possible each week to pay off the arrears so we don't get kicked out of our home.
I've had to call the police out to the house when he's turned up drunk, and started shouting at me and threatening to take my son away with him. It's a huge mess, and I don't know what to do anymore. I can't keep letting him mess the children around, my son is getting to the age now where he gets excited about seeing Daddy, only to be let down.
I was wondering if anybody had been in this sort of situation before, and where I should go next. I think I need to start going down legal routes regarding access, would you agree? I really have no idea what to do.
My grandad's funeral is next wednesday, and I am terrified my ex will let me down and not have the kids, and I wont be able to say godbye to my lovely Grandad.
Sorry if that was all a bit confusing, if you want to know anything else please ask, and be completely honest with me. Thankyou.