Since you have the Catalina installer, you could create a bootable USB stick (or SD card, I presume), set that as the startup disk in preferences (I think 
), then boot from the USB, install to the SSD, etc.
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Excellent suggestion, but one which was well-described in the Macworld article I referenced but which the OP missed or did not bother to read. 
I managed to get a Mojave installer from that GitHub link you posted which would install on the SSD. Thank you!
For some reason I cannot fathom and without any intervention from me, when the Mac Pro boots it seems to now favour this install rather than the HDD.
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Thanks again John, but I think the previous post from Ulli has nearly got me to where I need to go
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I did read the article. After creating the installer on the USB it states
- Hold down Option/Alt and press the Power button.
As ai have said 3 times, including in my first post, boot options are not something that is accessible to me
Did you ever explain why you updated the boot ROM? Was there a problem, or did someone recommend it, or did you do it on your own without understanding the repercussions?
Does the provider of your new firmware have any useful updates or a way to downgrade to the stock boot ROM?
Recall that there is a Recovery partition on the hard drive. The firmware works with that specific version of the Recovery partition to provide boot options (among other things)
As you have “replaced” the firmware and hard drive you have no boot options, as they need to be in sync.
If you match your version of firmware with that version of the operating system (that the firmware is for) and do a bare metal install after booting from an external device (usb, network, etc) you should be able to get them in sync. This assumes that your “upgraded” firmware is
“officially “ supported on your machine and you are not using something like Open Core Legacy Patcher.
Yes, I would be curious as well
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As far, as I figured out while I was searching for a solution:
The MacPro had an Apple Graphic Card while it was delivered. And a lot of people changed this Graphic Card with an alternate one.
For some reason it seems that this alternate graphic cards are having trouble to show the Bootmenue, but staying blank while it should show up.
There seems to be some specific chip on the Apple Graphic cards, that are missing on the third party ones, and therefore it is not working, as it should. (Maybe something similar to the replacement ID-Buttons for some iPhones?).
What I did not figured out, if this problem only occurred if you have an alternate graphic card installed, AND the new firmware, or if it is just an accidental timely coexistence of the two things.
It seems, that the new firmware with the Apple Graphic Cards do not show this behavoir.
Ahh, so new firmware with non-Apple graphics card.
(Was the new firmware installed to support the non-Apple graphics card?)