The annual increase is based upon RPI rates which can be found on the following Government website

http://www.statistics.gov.uk/cci/nugget.asp?id=19

RPI inflation rose to 4.4 per cent in June, up from 4.3 per cent in May. The main upward pressure came from mortgage interest payments – which are excluded from the CPI – with lenders passing on the remainder of this May’s quarter point increase in the Bank Rate. Other factors impacting on the RPI were similar to those that affected the CPI.