The other way to do this is - You would set April as the base of 100 so it would work in the same way.
If April is 100 and then May would be 1593.66/1457.92*100 to give you an index number of 109.3105. For September's index number it is (109.3105-100)=9.3105*5+100= 146.5526. So the amount for September would be - 146.5526/100*1457.92
Comments
If you work out the difference between April-May and May- June it is the same amount of £135.74 so month on month it is increasing by £135.74
Therefore to work out September you would so £135.74*5 plus April's figure of £1,457.92
Hope this helps
If April is 100 and then May would be 1593.66/1457.92*100 to give you an index number of 109.3105. For September's index number it is (109.3105-100)=9.3105*5+100= 146.5526. So the amount for September would be - 146.5526/100*1457.92