[ubuntu] Dual boot with Windows 10 stopped working

When I first installed Ubuntu 15.10, the ability to boot back to Windows from the grub menu worked, but it has stopped now. I can still boot into Windows using the BIOS to boot from the Windows partition, but when I try from the grub menu, it gives errors (implying I think that something in the Windows boot manager has changed or is broken.

This might be because I altered the grub menu using an app called grub customiser. I didn't specifically alter the entry for the Windows 10 boot, but I did change some colours etc, and also tried to write it to the MBR of the hard disk /dev/sda, which gave an error.

Welcome suggestions on how to fix (some sort of repair process with Windows)?