If you fight for your notice pay, then you may get it later on which is no use to you when your actually needing it now, cause the old boss wont hand it over easy.
I would be going in to see him for a friendly visit and discuss it with him.
I know with my job there is a consultation period of so many days if your contract is to be changed so the employer can discuss it with you, so if your old boss has just changed your contract there and then to suit himself then that's not allowed, but its as Dave says, its whether you want to fight it or not.
Its not easy when little ones are involved and I know the feeling.
My 1st employer when I worked at Partco after 5 years called me into his office out the blue one Monday and told me to leave instantly as he didn't like the fact I had been accepted into the army, even though I still had 4 months to wait until my Army entrance date arrived, and I had a 3 month old daughter, wife and house to support, and my parents had just passed away aswell, times were not good ... I went to dole to claim but then the dole refused to give me money as my old boss had told them I had voluntarily left my job !! ... wanker. The dole never did support me to keep family going, I ended up getting a job working nightshift for 4 months in a taxi office for £80 a week.
I always thought it was shocking that this boss was willing to see us starve, i'll meet him one day.