A woman who only joined us three weeks ago is claiming that she should be paid the same as a man doing the job. Can she do this?

There is no minimum service requirement for making claims under the Equal Pay Act. Her chances of success will depend upon the extent to which the jobs are similar - does his, for instance, include any extra responsibilities, or require any extra experience - and what, if any, are the material differences that justify you in paying him a higher rate. Take legal advice.