Nasko
Thank you for your reply. It turned out that this was a data issue - the user had entered a trailing line feed character in the field being bound to the 'DisplayValue' property of the parameter for a small number of records. The presence of a line feed character in this field appears to prevent the 'MultiCheckBoxValueEditor.prototype.UpdateState' method in the Telerik ReportViewer code from correctly identifying that the value had previously been selected. Removing the line feed characters from the affected records fixed the problem.
Regards
Paul