You also need to tell it to use a button to trigger the popup using the showOn setting. Set it to 'button' to only popup on the button click and not on focus, or 'both' to show on either.
('#MembershipWizard1_Wizard1_ctl05_ClubConfirmation1_txtStartDate').datepicker ({ dateFormat: 'dd/mm/yy',gotoCurrent: true,minDate: 0, showOn: 'both', buttonImage: 'App_Themes/Images/Ecommerce/datepicker.gif', buttonImageOnly: true });});