In order to qualify for Statutory Maternity Pay (SMP) your need to have been employed continuously for 26 weeks into the 15th week before the baby is due. You need to meet this criteria as well as other conditions such as your average earnings exceeding a certain level amongst other things.
If you qualify you would be entitled to 90% of your average weekly earnings for the first 6 weeks of maternity leave, then �123.06 per week (for 2009/10 tax year) for a further 33 weeks. These payments would be gross and subject to the usual tax and national insurance.
These payments are statutory and if qualified to receive payments subject to meeting all the conditions your employer must pay them. Some employers do offer Occupational Maternity Pay (OMP) which would be stipulated in the contract of employment or related terms of employment which you can check before signing a contract.
If you do not qualify for SMP your employer must issue form SMP1 which you could then use to assist a claim for Maternity Allowance.
The following website address will give more background on all of this.
http://www.dwp.gov.uk/lifeevent/benefits/statu tory_maternity_pay.asp#caniget