You have a right to be transferred from night shift immediately under UK health & safety law, if you can get a doctor or midwife to sign a note. If this cannot be accomodated they must send you home with pay.
Quote:"MHSW also states that where a new or expectant mother works nights and provides a medical certificate from her GP or Midwife which says that working nights will affect the health of the woman, then the employer must suspend her from work, on full pay, for as long as necessary. However, the Employment Rights Act 1996 provides that where appropriate, suitable alternative work should be offered, on the same terms and conditions, before any suspension from work."
http://www.hse.gov.uk/mothers/law.htm
DO NOT come at this from the 'I've got kids aso can't work odd hours' angle or you'll lose. Tackle it through health & safety, and good luck.