Averaging 2 values based on an ID in SSRS
I managed to do this in the end by altering the SQL query on the Dataset

SELECT        txtSchoolID, intNCYear, intReportCycleID, intSubjectID, txtCurrentSubjectReportName, AVG(CAST(intGradeTransposeValue AS decimal(5, 2))) AS avg_attainment, txtReportCycleName, txtGradingName, txtGrade, txtReportTermName, intReportCycleTerm, txtReportCycleShortName, dtReportCyclePrintDate, intGradeTransposeValue
FROM            VwReportsManagementAcademicReports
WHERE        (intReportCycleAcademicYear = 2017) 
AND (intNCYear &gt; 6) 
AND (intGradeID = 1)
GROUP BY intSubjectID, txtCurrentSubjectReportName, txtSchoolID, intNCYear, intReportCycleID, txtReportCycleName, txtGradingName, txtGrade, txtReportTermName, intReportCycleTerm, txtReportCycleShortName, dtReportCyclePrintDate, intGradeTransposeValue
ORDER BY intReportCycleID, intSubjectID