Neil,

Looking at the stack trace, the actual error is occurring inside of FillDataSet():

OakLeaf.MM.Main.Business.mmBusinessObject.FillDataSet(DataSet ds, String command, String tableName, String databaseKey, CommandType cmdType, IDbDataParameter[] dataParams)

Which is called from this line of your page:

mmSecurityAccessLevel AccessLevel = mmAppBase.SecurityMgr.GetAccessLevel(Session["mmUserSecurity_UserPk"],this.lblSecure.ControlID);

It's hard to tell from here what's going on. Can you set a breakpoint and determine exactly what's happening in these methods?

Regards,