Bootcamp partition size availability doesn't match storage space
I am attempting to create a Windows partition but Bootcamp assistant is saying that I don't have as much storage as I should. It says I will have 38GB free after 40 GB is used for Windows i.e. 78GB, but Storage shows 235Gb of 500GB of the SSD used i.e. 265GB free
Where is the missing 187GB??
A few people have had issues with Time Machine local images, but the suggestions posted to those questions haven't solved it. Terminal command sudo tmutil listlocalsnapshots / reveals no local images for deletion that I can see.
My Time Machine backups are currently on an external SSD but I did used to do them on a Time Capsule which stopped working for backups (reason unknown!)
Any help much appreciated - it's very frustrating!
iMac 27" 5K, macOS 10.14