Integrate a sensor

Configuration
1

I am scraping a value for rainfall in the last day from this site Rainfall at Hallaton gauge - GOV.UK which is sensor.rainfall_last_24hr
I want to integrate this to get a value for cumulative rainfall for this year.
The value is published every day in the morning, so I’m using a template sensor to add the new value to itself just before midday thus:

- trigger:
    - platform: time
      at: "11:59:30"
  sensor:
    - name: "Cumulative Rainfall"
      unique_id: cumulativerainfall
      unit_of_measurement: mm
      icon: mdi:sigma
      state: >
        {{ states('sensor.cumulative_rainfall')|float |round(1) + states('sensor.rainfall_last_24hr')|float |round(1)}}

Can anybody advise the best way to do this? TIA