I directly bought some stocks in my ISA, all with roughly similar allocations.
If I wanted to start a pie instead, containing all the same stocks (reason being the ease/automation of allocation), is there any way to ‘transfer’ my Investment holdings to the Pies section, or must I sell my stocks in order to invest the cash into a pie?
Thanks!
Welcome to the Community! 
If you want to move your shares into a Pie, you can import them directly. To do so, you can follow the steps mentioned in this article.
Nevertheless, if you need any additional assistance, feel free to drop me a DM 
Is there a way to transfer some of my investments directly into a pie using the web based version?
…thanks
Yup, you can transfer shares from your portfolio to a specific Pie through the web app as well.
To do that, select the “Pie” icon located underneath the “Home” icon on the web app. Once you open the tab, go to Pies > Select your preferred Pie > Holdings, and you can find the Import option at the bottom of the screen.
From there, you will be able to select which shares you want to import from your Pie and confirm the action. Let me know if you have any other questions here, or feel free to drop me a DM at any time 
Thank you for the quick response 