Hi BackToAus,
To claim split year treatment, requires the completion of a Self Assessment tax return.
The SA100 (tax return), SA109 (residence) and any supplementary pages required to declare your UK residence income and gains.
The HMRC online tax return, does not support the completion of the residence section and cannot be used to submit the tax return online.
If you have a government gateway user ID and password, you can buy a commercial tax return that supports residence and submit that version online.
A list of commercial suppliers can be found here:
Self Assessment commercial software suppliers.
If you cannot verify your identity to obtain a government gateway ID, you will need to download and print off a paper tax return here:
Self Assessment tax return forms