I'm also not going back to work once my maternity leave is over - not by choice though as my contract ended.
But in terms of smp, I am getting paid my full amount even though it will continue 5 months past my contract ending, and I don't have to pay any back. I think every job is a bit different, especially if you work in the private sector.
I've heard different things about smp, my manager says i'd have to pay it back if i was to leave before the 6 months or materinity is up but i can't find anything about that one any sites i have looked at. I work for a farily large company, i think there 300 400 odd stores in the uk so i wouln't have thought i'd have to, i think i'll have to talk to my mangaer about it at my next risk assiment.
Do you have a human resources department you could contact to clarify? For me, I get SMP as well as work contributing as after so many weeks top top up the money as the pay goes down - if I leave I have to pay that (the top up money) back but I can opt to not take it and if I do go back I get a lump sum at the end of my leave. That's how my work has done, I hate in a way that it's all different and it so confusing! I would double check, plus, even if you intend to leave, you don't have to hand in your notice now, im sure you'd be better off taking the SMP then handing it in rather than leaving. Defo get some advice about it from others tho, not just your manager just to be sure on the government entitlements not just the work ones. Have you already handed in your MAt B form?