TLSM is it Maternity Allowance or Statutory Maternity Pay you are getting?
You won't have to pay either back, but just to get a bit of clarification.
The only maternity pay you would ever have to pay back is if your boss is paying you anything over and above the statutory basic, and has put something in writing saying you hvae to return to work for x amount of time ohterwise you have to pay it back.
Normal SMP or MA you don't hvae to pay back, that's yours now, you've already qualified for it and nothing you can do will change that.
Your boss should certainly not be harassing you on maternity leave. You can (if you are happy to) working up to 10 days during your maternity leave without jeopardising your maternity pay, so if you work any days (which is up to you) make sure you get paid for them on top of your maternity pay.
It might be tempting to resign now, and you would still get your maternity pay anyway, but don't do this. During your maternity leave you are entitled to holiday as well. So if you choose not to go back at the end of your year off, your employer will have to pay you however much holiday pay you are due for the whole of your maternity leave, assuming you didn't take it before you left.
Certainly don't do everything he says. Don't resign now but tell him strongly that you are not working at the moment and he or the temp must not contact you during your maternity leave about work -related issues.
Quite frankly, if the temp is rubbish, that's his problem not yours, he should recruit someone better!
If you are sure you are not going back you needn't worry about annoying him by telling him to leave you alone, or by just ignoring the emails, and you won't lose any money by doing so. The only thing you stand to lose if you resign now is holiday pay, but other than that you can be as firm as you like with him about the contact.
You are entitled to a year off as you know. Assuming your notice period is a month, write to your boss a month before your year is up handing in your notice and telling him you won't be returning. That way he will have to pay you all your holiday, and 4.8 weeks of holiday is definitely worth it if you can manage not to tell him until then.