If you were on Jobseekers when you went on to Maternity Benefit, you can pick up where you left off once your Maternity leave/benefit expires, provided that there is not a period of more than 6 months between stopping your Mat Benefit and your new claim.
Once you exhaust your Jobseekers Benefit, you may re-qualify if you have the stamps (you need 13 stamps after day 156 of your claim, so you'll more than likely meet this criterion), although you would also need to have lost a day's employment, so if you continue on 3 days, you'll not have any entitlement to re-qualify for Benefit.
If you don't qualify you may qualify for Jobseekers Allowance, although this is means tested.