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As a night worker your wife should not work more than eight hours in a 24 hour period. There are exceptions to this and you may like to check out http://www.direct.gov.uk/en/Employment/Employees/WorkingHoursAndTimeOff/DG_10028519
to see whether these apply to her.
However as she is pregnant, she is entitled to ask to be transferred to suitable day work, or if none is available to be suspended on full pay. Your wife should declare in writing that she is pregnant. The employer should in any event carry out a risk assessment as part of the health and safety of a pregnant woman working for him.
If the employer does not agree to give her day time hours or suspend her on full pay, she can make a complaint to her union or employee representative and if this does not work to the health and safety executive. She could also complain to an employment tribunal.
This link may be useful to you and her. http://www.hse.gov.uk/mothers/
I hope that helps.