Error with shim-signed package when running updates

When I run 'apt upgrade', 'apt update' etc, I get the following error:

Setting up shim-signed (1.40.10+15.8-0ubuntu1)  
Mount /var/lib/grub/esp: special device /dev/sda1 does not exist  
Dpkg: error processing package shim-signed

I think this fails because the /boot/efi partition in /etc/fstab is a UUID which links to /dev/sdc1 (which is correct). (/dev/sda is a raw data disk with no partitions) The file /var/cache/debconf/config.dat has the following settings:

Name: grub-efi/install_devices  
Template: grub-efi/install_devices  
Value: /dev/sda1  
Owners: grub-common, grub-efi-amd64, grub-pc  
Flags: seen  
Variables:  
    CHOICES:  
    RAW_CHOICES  

Name: grub-efi/install_devices_disks_changed  
Template: grub-efi/install_devices_disks_changed  
Value: /dev/sda1, /dev/sdc1  
Owners: grub-common, grub-efi-amd64,   
Flags: seen  
Variables:  
    CHOICES:  
    RAW_CHOICES

My questions are: Can I just change the config.dat file to use /dev/sdc1? Is there a command to do this rather than just edit the file? What's caused this to happen in the first place when there are identical servers which behave correctly? I'm wary about reinstalling grub etc as I don't know if this will do anything with /dev/sda. Any help appreciated please.