I then tried to do this via partedUtil but encountered a "Read-only file system during write on" error:
I verified the device wasn't being used for any existing datastore's, DRM's or diagnostic logs:
I remembered that this device was previously used for VSAN so checked that it still wasn't a member of the cluster, which it wasn't, but the devices were still associated with the disk group somehow:
Problem was resolved by using the "esxcli vsan storage remove -u <VSAN UUID>" to disassociate the device from the VSAN Cluster:
Once this was completed the devices were no longer in use and I could clear the partitions and create a local datastore.