Keely,
You only need to claim Maternity allowance if you are either out of work or have not worked for your employer for more than "25 weeks before the 15th week that the baby is due".
Maternity allowance is paid by the job centre and is £108.85 p.w. ( you must have worked more than 26 weeks out of 66 with average weekly earnings of £30.00 or more if your average weekly earning is less than £108.85 then you are only entitled to 90% of average weekly earning ) no NI or tax to be paid as you are considered "unemployed" but it is counted as income against benefit claims though not against tax credits. If you haven't fulfilled the criteria you can still make an appeal.
If you have been in work for longer than "25 weeks before the 15th week that the baby is due" continuously, it will be SMP paid by your employer (you must earn above the LEL (lower earnings limit of £84 p.w. which means that you have made NI contributions - otherwise claim maternity allowance). Many employers have their own policies but by law they have to offer their staff at least 6 weeks at 90% of salary (no upper earnings limit) and then 20 weeks at £108.85 or 90% of average weekly salary if less than £108.85. You will pay tax and NI same as you do with normal wages I'm afraid but the £108.85 is net of tax, this is your take home figure.
I have worked in payroll and it took me ages to get my head around this, I hope I have got it all right and that it makes some sense!
Kat x