I am currently on mat leave too and was told by my manager, everyone is entitled to a year off, normally the last 3 months are unpaid. In some places if you don't return to your job you have to pay back some of your maternity pay and work your notice. Do they know you might not be going back? Try and find out the policies and procedures regarding what you have to do, then go from there. If you dont have to pay it back and work your notice then maybe hand in your notice at least 4weeks before your maternity leave runs out. This is all I know for the company I work for, hope it makes sense lol
Your statutory maternity pay should be unaffected. If you get any extra from company you are likely to have to pay it back unless you return to work for a certain length of time. What does your contract say?