As you only work one day a week, you are not (as I'm sure you are aware but for other's benefit) entitled to SMP.
However, you probably qualify for Maternity Allowance.
You must have been in employment for 26 of the 66 weeks up to your due date to qualify for Maternity Allowance. In addition, your average weekly earnings for 13 of those 66 weeks has to be at least �30.
You can choose which 13 of those weeks are used in calculating the average, so you can choose the highest paid weeks.
The fact you have quit your job doesn't stop you from claiming MA. However, as you were working for them in the 26th week of pregnancy, you should have received a form SMP1 telling you that they couldn't pay you SMP. You will need this to claim MA. You can get a claim form from the Jobcentre or download one from
http://www.dwp.gov.uk/advisers/claimforms/ma1. pdf.
You are entitled to claim MA from 11 weeks before your due date, so get your claim in straight away as you could already have been getting the money!