Can't create partition in Boot Camp

I'm trying to install Win10 in Boot Camp on a new iMac. I've downloaded a Win10 1803 ISO and run Boot Camp, which gets as far as partitioning the disk and then hangs; the blue progress bar doesn't move at all. I'm trying to create a 350g partition for Windows on a 1TB drive.

I eventually Force Quit out of Boot Camp and had to recover the space set aside for the new partition using repairDisk in Terminal.

Output from diskutil cs list:

And diskutil list:

All help gratefully received!