High Sierra OS 10.13.3 restart error

Installed High Sierra OS 10.13.3 on 31 January 2018 on my 2010 iMac, processor 2.8 GHz Intel Core i5, 8 GB DDR, 1TB SATA Disk (792 GB remaining). Apart from extreme slowness there's been no other problems until restarting computer today (14 March 2018), and looped into this message: "the path /System/Installation/Packages/OSInstall.mpkg appears to be missing or damaged. Quit the installer to restart your computer and try again." Repeated restarts take me back into this message.

Tried startup while holding Option, Command, R, and also tried startup while holding Option, Command, P, R, with no success. Have now gained access in Safe Mode only. Would like to access Disk Utilities to reveal the problem, but can't find it!

Any ideas on how can this startup problem be resolved, or how to access Disk Utilities?