I've had a reply from the student union, copied below for other people's information. The final link is quite useful. Apparently Research Councils often pay a maternity allowance and other PhD sponsors sometimes do too.. worth checking out!
" Thanks for your email.
As I am not qualified to advise you directly in relation to whether or not you are eligible for maternity pay, I am providing you with some links which I hope you may find useful. The main website that provides information about maternity leave in the UK is DirectGov - www.direct.gov.uk/en/Parents/Moneyandworkentitlements/WorkAndFamilies/Pregnancyandmaternityrights/DG_10039631.
Cambridge University's Childcare Office has some information on their website about maternity benefits - www.admin.cam.ac.uk/univ/childcare/student/finance.html#maternity
Also, AMOSSHE (the Student Services Organisation) has provided some guidance for students about maternity pay - www.ecu.ac.uk/publications/files/student-pregnancy-and...for.../file. Please see extract below.
Maternity leave and maternity pay
If a student is employed or has recently left employment, she may be entitled to statutory maternity pay, additional maternity pay or a maternity allowance. Further information can be found at www.direct.gov.uk.
Students who receive a stipend, NHS bursary or research council funding are often entitled to maternity leave and maternity pay from their sponsor. Students who are entitled to a stipend or grant will need to discuss with the appropriate HEI contact whether the HEI needs to inform the sponsor of their pregnancy, and should familiarise themselves with their sponsor?s maternity leave and pay policy.
Based on the paragraph above, it may be an idea to check with your funding body to see if you may possibly be eligible for maternity pay from them.
I hope this is helpful. "