Understanding a clause and CPI help please

[JABB]

I have a clause as follows:
22. The husband shall pay periodical payments to the wife for the benefit the child of the family at the rate of £400.00 per calendar month payable monthly (less any sums up to £400.00 per calendar month of any child maintenance assessment that is paid by him). Payments shall continue until the child of the family attains the age of 18; or ceases full-time tertiary education, whichever shall be the later; or further order.

23. The figures in paragraph 22 shall be varied automatically on the anniversary of this order and at yearly intervals afterwards ("the Variation Date") by the increase (if any) between the retail prices index for the period of 15 months before the Variation Date and 3 months before that date

Now, I have looked at this : https://www.ons.gov.uk/economy/inflationandpricein... but for the life of me, I don't understand what I am supposed to be doing. Please can someone advise what I should be increasing on year one anniversary ( 1.9.16 and year two 1.9.17 )

I would like to understand how to work it out so I can go on myself.
Thanks guys

[AyBee]

It sounds like it's RPI, not CPI, that you need to be looking at. Essentially, £400 will increase with inflation (RPI), but I wouldn't have thought you'd have to calculate that yourself and somebody will tell you what the revised amount is.

[JABB]

No one has said anything until this week when the ex asked to top up the amount. I wanted to work out what it should be rather than what she wants.

[Mandat]

Based on the RPI index from the ONS link, the RPI figures between June 2015 to June 2016 increased by 4.2. This equates to a 1.596% increase, therefore on 1 September 2016, the payments should have increased by £6.38 per month to £406.38.

For the 2017 adjustment, the RPI figures between June 2016 to June 2017 increased by 9.2, equating to a 3.378% increase. Therefore on 1 September 2017 the payments should increase by £13.73 to £420.11 per month.

Hopefully, others will be able to verify or not my above calculations.

[JABB]

Mandat said:

Based on the RPI index from the ONS link, the RPI figures between June 2015 to June 2016 increased by 4.2. This equates to a 1.596% increase, therefore on 1 September 2016, the payments should have increased by £6.38 per month to £406.38.

For the 2017 adjustment, the RPI figures between June 2016 to June 2017 increased by 9.2, equating to a 3.378% increase. Therefore on 1 September 2017 the payments should increase by £13.73 to £420.11 per month.

Hopefully, others will be able to verify or not my above calculations.

Thank you so much!