They’ve just got back to me and asked me to run:
tmutil deletelocalsnapshots /System/Volumes/Data
Which has dropped my used space to 345.76 GB
And separately:
- 46.83 Applications
- 1.21 GB - Library
- 28.16 GB - System
- 268.82 - Users
Looks like you were right about the snapshots @NobbZ 
Marking the as the solution in case it helps anyone else in future…