Is a will still valid if the person making it remarried and then got separated after making it in the UK?

My father made a will first when he married my mother, then again after I was born, and then made another one after he divorced my mother, which made me inherit everything except the £30K he owes my mother. However, he then had two other children, and remarried. But he was separated from his wife for a year before he died. I was just wondering if that makes his will invalid, and if it does, does my ex-stepmother still get two-third of the remaining estate (after the £30K he owes my mother)? I have absolutely no problem sharing it up between my siblings and I, but I'd rather resent it if my ex-stepmother still got two-thirds of the estate. Also, my siblings are only 10, what happens to their share of the money? I am 22, and in University full-time.