@PhilPhil7000 I can’t really help you much with the location of the directory.
All I can suggest is to make note of the file/folder names in the directory and then search EVERY partition in Windows, even those that have not had a drive letter assigned to them and see if they match what you see in the Dell BIOS.
Through trial and error you should be able to eliminate each partition one by one.
[Edit] Once you think you have found it, create a new folder with an obvious name like ‘EFITEST’ and see if it appears in the BIOS directory.
OK thanks @chinobino. I know windows creates lots of partitions on a drive. What tool would be able to see the hidden ones? Would the windows 10 USB stick let me see or will I need a Linux boot key?
@PhilPhil7000 For hidden partitions such as Windows Recovery Partition (NTFS) and EFI System Partition (FAT32) it may be possible to to unhide/mount them them with diskpart but booting to a Linux USB/live CD may well be easier/quicker.
Hi @chinobino
I managed to copy the shell you recommended and the ui64.efi file to the hidden windows partition.
I can now boot into the Marvell Bios pre windows boot if I ever need to without any USB Keys.
I have updated my post above with the full steps.
Hopefully if anyone buys one of these cards or the similar startech model, it will give them a way to future proof their installation.
Thanks again for all your help, couldn’t have done it without you.
@PhilPhil7000 I’m glad to see your persistence has paid off, thanks for reporting back and detailing all of the steps taken. You’re welcome 
@chinobino Thx, i added a UEFI Shell to my Grub2 and copied the UI64.efi to the UEFI boot partition, now I have easy access to the RAID config. Very strang that I cant find a way via a PCs UEFI… does UEFI does not support 3rd party "BIOS" anymore?
BTW I tried to add the Marvell config tool direct to the Grub2 menu, but i wont show up. I guess the shell is needed.
If you are running in "pure" UEFI mode (CSM disabled) then legacy "BIOS" cannot be loaded. With CSM enabled you may be able to load certain legacy BIOS/firmware/option roms if the configuration option is present in the UEFI.