Parents can take parental leave after maternity leave and will be entitled to the same terms and conditions as if she was returning after additional maternity leave (52 weeks). This means she can return to the same job, where possible if not, a suitable alternative should be found. Parental leave is a separate entitlement for employees who have worked for the company for one year and must be used to care for the child or to find suitable childcare arrangements. Each parent can take thirteen weeks for each child and it is unpaid. If you have twins, this means thirteen weeks for each twin.
If you do not qualify for parental leave, you could take paid holiday or ask your employer for unpaid leave. It may be worth discussing flexible working options with them, too.
See: Work and families act (2006) London. HMSO.