some companies pay above the rates you mentioned - I take it your company doesn't, and no company maternity policy available for clarification? My HR department were kind enough to send me a schedule of payments to clarify things for me, might be worth asking something similar of your company.
The best place to get information about maternity pay is from the direct.gov.uk website. The site is great for finding out anything to do with the government.
Look at this page for information on maternity pay, including a really useful tool that will produce a personalised statement of what you will be entitled to, although it will not include anything over and above the minimum requirements that your company may pay you, which is why you should check with them about your entitlements.