I suspect male colleagues doing the same job are being paid more, how do I go about finding out?

You have the right to ask via the new Equal Pay Questionnaire that was introduced by the Government in April 2003. For a copy of the questionnaire, and guidance on filling it in, see: www.womenandequalityunit.gov.uk/pay/update_question.htm

The questionnaire explains that your employer can refuse to complete the questionnaire, but if they do so and you do choose to take the questionnaire to a tribunal, then 'inference can be drawn' from their refusal. So the sanction only comes if the case goes to a tribunal.