In order to receive Statutory Maternity Pay, the employee must, at a point 15 weeks prior to the birth of the child, have worked for the employer for a minimum period of 26 weeks.
If an employee does not qualify for Statutory Maternity Pay she may qualify, instead, for Maternity Allowance. This is payable if, at the end of the week prior to the expected date of birth, she has either been employed (by anyone) or self-employed for at least of 26 of the previous 66 weeks.
Therefore, if your fiance has been continuously employed with her current employer for 5 years and becomes pregnant shortly after changing jobs, she won't qualify for SMP but will, instead, qualify for MA. (For most of the period during which these benefits are paid, the payments are exactly the same).
http://www.dti.gov.uk/er/matleafr.htm
Chris