Been charged with ABH ( Actual body harm), what will the punishment be ?

firstly get a solicitor, you should have had one when you were questioned. but as it's a 1st offence you will probably get a fine, or community service. you need your solicitor to stand up and explain the reasons for your behaviour to the judge or magistrates. in your case, as the guy you attacked had a knife, i don't blame you for your actions. but as the judge will probably say to you, you can't take the law into your own hands. so basically, you will just get a reprimand. and it's your word against the other guy that he had a knife. but you need a solicitor to explain it all on your behalf, as if you are anything like me, when i'm under stress i get all my words muddled up! don't worry, you will still be sitting down to xmas dinner - at home - but good for you i say, at least you were honest. it just shows what a coward the other guy was, not only carrying a knife, but going crying to the police. in my day, in about 1845! people used to have the odd fight and nothing would come of it, now they go crying to the authorities trying to get comp.