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 an Equal Pay Questionnaire. For a copy of the questionnaire, and guidance on filling it in, see: www.equalities.gov.uk/docs/Equalpayquestionnaire.doc

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.