Writing to a file where the filename is held in a TC property

When looping through a Datasource and parsing Rest requests I'm trying to create a file on the 1st iteration and write to that file on all iterations.

I'm storing the file name in a TC property and when debugging the code it is objecting to the line highlighted below saying I cannot write to the file defined like this.

I'm a Groovy noob so any help would be really appreciated

def firstTimeThrough = testRunner.testCase.getPropertyValue('firstIteration')

fTime = firstTimeThrough.toBoolean()

log.info(fTime)

if(fTime == true){
	def date = new java.text.SimpleDateFormat("yyyy-MM-dd-HH-mm-SS")
	def fname = 'FRDealerResponseFile_' + date.format(new Date())
	def groovyUtils = new  com.eviware.soapui.support.GroovyUtils(context)
	def projectPath = groovyUtils.projectPath
	def resultsFile = new File(projectPath + '\\Results\\FR-Dealer-Get-Products\\' + fname + '.log')
	testRunner.testCase.setPropertyValue("externalResultsFile", projectPath + '\\Results\\FR-Dealer-Get-Products\\' + fname + '.log')
	testRunner.testCase.setPropertyValue("firstIteration", 'false')
	testRunner.testCase.setPropertyValue("overallResult", 'PASSED')
	log.info('FIRST TIME THROUGH')
	testRunner.testCase.setPropertyValue("firstIteration", 'false')
}

// Debugger objecting to this line
resultsFile = testRunner.testCase.setPropertyValue("externalResultsFile")
//

log.info(firstTimeThrough)

obj = context.testCase.getTestStepByName("Get-FR-Dealer-Products")
assertions = obj.getAssertionList()
assertions.each{
stat = (it.status).toString()
log.info (stat)
   if(stat == "FAILED"){
     log.info "FOUND FAILED"
	resultsFile.append(it.name + ' --> ' + it.status)
	res = 'FAILED'
   }
}

Here is the error message returned by the debugger

groovy.lang.MissingMethodException: No signature of method: com.eviware.soapui.impl.wsdl.WsdlTestCasePro.setPropertyValue() is applicable for argument types: (java.lang.String) values: [externalResultsFile] Possible solutions: getPropertyValue(java.lang.String), setPropertyValue(java.lang.String, java.lang.String) error at line: 21

Mark Smith