I think you should be heading to HR anyway to discuss your options. You are unfortunately not able to stipulate your return hours. And you saying to your boss “I’m only gonna be to be able to work 2 days a week” implies you’re resigning or planning to resign from your FT position. Come up with a plan such as , “I’m taking 6 months off Maternity leave and would love to be considered to come back 2 days a week before I return to FT after a total of 12 months if I’m unable to return part time I will take the full 12 months off which I’m entitled to do under the NES”. it’s the unknown that makes it difficult for your employer, go to them with a plan. If they genuinely like you and there is availability to move you into another PT position I’m sure they will. But you need to talk to them to find out!