heard that they have changed The amount of Mahr written on the Nikah Namah (Marriage certificate), what action shall I take regarding both matters?

My husband has been away for 3 years, he won’t divorce me neither will he get back together with me! We have been splitting up and getting back together for the past 3 years as he has had a gambling problem, which I was unaware of and he left us (me and my children) with loads of debt.After being away for 5 months, we reconciled and I let him back into our house again to give our marriage a 2nd chance.I then got pregnant with my second son and after 5 weeks at home, he left again leaving me pregnant along with my 3yr old son. He then went back home to Pakistan, I did not have any contact with him for months, and through friends and family I was told that he had been sent to prison for sexually assaulting a girl whilst he was still living at home with me.

He wrote to me from prison, and asked me to visit him, to which I agreed to give the marriage another chance. When he came out of prison he did not come home (as I thought he would), instead he got himself a flat in another city and did not have any contact with me. After 3 months, he came down with some family members to see if we could work things out. My youngest child was then 6 months old, he said he would live in another city, and come and go every weekend as he was planning to rent a house for us there. He came every other weekend for the first 2 months, and then cut off all contact with us. I then heard he has gone to Pakistan yet again, I contacted him to ask what he planned to do, as I live alone here (uk) with my two children. He said that, I should book my own ticket, sell everything that I own here, (house, car etc,etc), pay off his debts here and join him in Pakistan! I said to him if he wanted me to go over there he is my husband, and therefore he should send me and my children tickets to go over there, as he does not pay any money for me or my children for the past 3 years. Please could you advise me on what step to take next as I have given him many chances, he does not want to get married again neither will he divorce me, and I can not carry on living by myself with  2 boys, (one who is 6yrs and one who is 15months). I have also, heard that they have changed The amount of Mahr written on the Nikah Namah (Marriage certificate), what action shall I take regarding both matters?