As per the law, you are eligible for maternity leave. Going through a lawyer is expensive and, in the end, counterproductive because of time, efforts, cost, and the fact that your career thereafter gets affected. No employer wants an employee who has gone to court.
You can show them the law and tell them that you are willing to resign, but they should pay 12 weeks of maternity leave. If they agree, good. If not, you need to decide how much you want to invest in a fight. If they are giving you a month's salary as notice pay, why are you worried about them not paying? Do they have a habit of bluffing or going back on their words?
You must also remember that any stress that comes during this time is bad for the health of you and your child. So, you need to weigh the economic parameters and health and decide if you can afford to just walk off with a notice period and look for a fresh job later.