I subclassed the Check Box List class to add the Validation Property that the Required Field Validator is looking for.
By Bill Evjen Reuters December 2003 Applies to: Microsoft® ASP. NET validation server controls that are at your disposal.
) error CS1061: 'ASP.common_controls_specialreportform_ascx' does not contain a definition for 'validate Check Boxes_Server Validate' and no extension method 'validate Check Boxes_Server Validate' accepting a first argument of type 'ASP.common_controls_specialreportform_ascx' could be found (are you missing a using directive or an assembly reference? user1463201, can you confirm whether the checkboxes are on the parent ASPX page and the code is behind the ASCX page (as per your question), or is that a typo and the checkboxes are also on the ASCX page? Actually, I had a typo, which I have edited in my original question.
The checkboxes are on the page, and the validate Check Boxes_Server Validate function is in the code-behind, on the cs page.
In the spirit of asp.net's validation controls can I use to enforce this. Give all your checkboxes a class name say 'reasons'.