I have finally got everything sorted!
That was a marathon. You're right EasyBCD isn't compatible with UEFI. EasyUEFI (Free trial) was excellent for what I needed. After flashing BIOS a couple of times, the boot failed to load the windows boot manager either, so I was left in the grub cli.
So, in the end I needed to perform these additional steps:
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Run boot-repair from the Zorin Live USB. (this corrupts BIOS)
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Reboot into installed Zorin.
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Install rEFInd to replace grub as boot manager.
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Reboot into Windows.
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Reflash the BIOS using Fujitsu's utility (this fixes BIOS)
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When it loads into grub cli, manually boot into windows efi, like this, but replace hd0,gpt1 with the correct partition:
insmod part_gpt
insmod chain
set root=(hd0,gpt1)
chainloader /EFI/Microsoft/Boot/bootmgfw.efi
boot
When Windows loads:
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Run EasyUEFI
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Click Add Item > Select Linux or other OS > name it rEFInd
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Check the partition containing the rEFInd boot loader
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Browse to "/EFI/refind/refind_x64.efi" for the file path
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Click OK to create the rEFInd entry in BIOS boot options.
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Move the entry to the top of the list.
Reboot and you're done!
I now have both a dual booting system with Zorin and Windows AND a BIOS that I can configure and boot from USB sticks , seems like such a simple thing at first....