Finally found work around.
Once installer started, I checked mount and mount target mounted ESP on sda.
Unmounted that partition & mounted ESP on sdc.
ubuntu-mate@ubuntu-mate:~$ history
1 mount
2 sudo umount /target/boot/efi
3 sudo mount /dev/sdc1 /target/boot/efi
4 mount
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https://bugs.launchpad.net/bugs/1396379
Title:
installer uses first EFI system partition found even when directed
otherwise
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