I'm due to go back to work end of April (mat pay ends then). I received SMP, no extras from employer etc.
I haven't said I won't be going back, to be honest I'm not really sure what to do. Do I have to give my employer a certain amount of notice that I'm not going back before my SMP ends? ie 6 weeks notice etc?
As long as you only received SMP then there is nothing to repay whatever you decide . You normally have to give notice as per your contract if you aren't going back or 8 weeks before your date of return, otherwise it will default to 52 weeks after ml began. You don't need to contact them just because SMP has finished, so can use the unpaid time to make your decision.
Its eight weeks notice to change your plans if you are staying on but changing the return date, but if you are going to resign, it's just your contractual notice same as if you were resigning and you weren't on mat leave. check your contract.