I've checked using both the built in driver
checker in Device Manager and by checking the version available on the Lenovo drivers
page, and I'm running version 18.104.22.1682 which it claims is the most up to date version.
I've tried to install other drivers from the Intel RST page, but it keeps saying I do not meet the minimum system requirements.
I then downloaded and burnt to CD the iso file on the Lenovo website called "Solid State Drive Firmware Update Utility", which confirmed that no firmware update for AHCI was needed.
I also confirmed in the BIOS that AHCI is indeed enabled.
I think these freezes are pretty much definitely due to the disk, as during them CPU usage drops to around 2-4% while disk usage is at 100%.