Advice On when/How to fire a script

Hi all,

I have a script, which basically dependant on a field value loads a related record captures some data and then populates it into a store field for displaying in a HTML field.

The script works fine in Debugger and does what I want it to but I just can;t figure out how to set it to run, I have tried FieldChanged and Pageinit but it just keeps saying that function is not referenced.

Any ideas advice please?

 var NewRec = nlapiLoadRecord('customrecord_report_writing',1);                  nlapiLogExecution('DEBUG', 'Execution Context', nlapiGetContext().getExecutionContext() );                 var getType = NewRec.getFieldValue('custrecord_rw_wpa_type');            //1=CGI,2=BT,3=LBG,4=CSC-HMPO,5=Individual@work,6=Test Record            nlapiLogExecution('DEBUG', 'Type', getType);            var WPAID = NewRec.getFieldValue('custrecord_rw_wpa_link');            nlapiLogExecution('DEBUG', 'WPA ID', WPAID);            var WPARec = nlapiLoadRecord('customrecord_wpa_20',WPAID);                        if (getType == 6)                {                var LBGPrimaryCondition1 = WPARec.getFieldValue('custrecord_wpa_lbg_primary_condition');                var LBGSecondaryCondition = WPARec.getFieldValue('custrecord_wpa_lbg_secondary_condition');                var Workingenviroment = WPARec.getFieldTexts('custrecord_wpa_wrkstat_environment');                var Workingenviroment = Workingenviroment.join('<br>');                var LBGJobRoleInvoles = WPARec.getFieldValue('custrecord_wpa_lbg_job_roe_involves');                var LBGDSECoducted = WPARec.getFieldText('custrecord_wpa_lbg_dse_conducted');                var LBGDSEDate = WPARec.getFieldValue('custrecord_wpa_lbg_dse_date');                var LBGDSEOutcome = WPARec.getFieldValue('custrecord_wpa_lbg_dse_outcome');                var LBGbackground = WPARec.getFieldValue('custrecord_wpa_lbg_background');                var LBGrepeatReferral = WPARec.getFieldText('custrecord_wpa_lbg_repeat_referral');                var LBGPreviousAdjustment = WPARec.getFieldValue('custrecord_wpa_lbg_previous_adjustment');                     var StoreFieldCode = ('<STYLE TYPE="text/css"> <!-- TD{font-family: Tahoma,Geneva,sans-serif; font-size: 8pt;} '                        + 'TH{font-family: Tahoma,Geneva,sans-serif; font-size: 8pt;font-weight:bold;} '                        + 'table.padded-table td