This guide is based on Mike Kasberg's guide so give thanks to him for the bulk of this. I've just customised for our situation and to avoid having to reinstall the Windows that we are provided with.
Here's broadly what we'll do:
- Disable Bitlocker on Windows
- Run the official Ubuntu graphical installer for (unencrypted) dual boot
- Delete the Ubuntu partition and replace it with the encrypted partition setup
- Install Ubuntu onto these partitions