According to the example
Sara joins the RAF Other Ranks in November 2013. She will get the FREA for 1 year (2013 to 2014). This will not be reduced even though her service started part-way through a year. No RPS will be paid as there has been no delay in making the repayment. She is a basic rate taxpayer.
2013 to 2014 £100 @ 20% = £20
Total Repayment due £20
So £20 per year if I read it correctly.....every little helps and that is a little!