Materntiy pay HR advice

I was hoping someone could help me and shed some advice on my situation. I have just turned into my 9th month on maternity leave and I am still getting paid my normal amounts. According to the maternity leave contract that I was sent by HR, I should have been on the standard SMP by now and I should have been since month 6. I haven’t been able to access any of my online pay slips since I’ve been on maternity leave, so I’ve no idea what my break down is… I’ve chased this up with the HR contact that sent me the letter and they haven’t responded to any of my emails. I’be been making sure to put aside the extra pay from the last 3 months away, ready to pay them back - I just don’t know what to do next, especially with nobody responding to me. Will they get in touch with me when they figure out they’ve made a mistake and ask me to pay them back? Weirdly enough it’s a horrible feeling, having this unexpected pay, I want to get rid of it because I know I shouldn’t have it - is there anything legally I should know about? What more can I do?!!