OK Thanks for all the helps guys! I've not got there totally - I can't get into clover on the HDD but I've managed to at least get to the point where High Sierra is installed and I can boot into it via the install USB (albeit with about a 20% success rate).
So, taking on board the posts in the links and erman1337 and BreBo's comments, I tried a whole bunch of stuff but this is the only thing so far that has got me as far as actually installing High Sierra:
1) Make a Sierra (Not high sierra) install USB (found from a friend that the conditions that meant I could only get a stub installer were that the OS was pre-sierra or not enough space. Since I'm using a blank 2TB drive I assumed it was the old OS issue. This may not work for everyone but it did for me.). For this I used the following link on my old Mavericks installation and uniBeast7. I used UEFI not legacy:
https://itunes.apple.com/us/app/macos-sierra/id1127487414?ls=1&mt=12
2) Installed Sierra. Configured With MultiBeast (on my mobo needed the Mausi Ethernet kext).
3) Downloaded the High Sierra installer from the AppStore (wonder of wonders this actually gave me the full file not the stub, I'm assuming because it's from Sierra rather than older.)
4) Used UniBeast 8 to create an install usb and manually removed apfs.efi from
/Volumes/EFI/EFI/CLOVER/drivers64UEFI/
(this is the only way I could get it to boot)
5) Installed without the apfs conversion as per the guide.
6) Configured with Multibeast using: UEFI boot, 3rd Party SATA, Mausi Ethernet (newest), 3rd Party USB3, Clover UEFI, Nvidia graphics fixup, as MacPro 6,1
7) Installed relevant nVidia web drivers.
This has given me an installation of High Sierra which seems to work (ish) with the notable exception so far that sleep mode causes a crash but I've just disabled sleep as I never use it anyway and it gets in the way of my normal work in any case.
My main problem now is booting.
I cannot boot directly to the HDD clover install (have also updated clover, same issues), with UEFI my bios tries to boot it but just asks me to install bootable media, and with legacy it opens the grey and blue boot manager which cannot see anything bootable either.
I can boot into the installation, but only via clover (4297) on the install usb, with the flags (after nVidia install) of nvda_drv=1 (nv_disable=1 before graphics driver install) and -v (so I can tell if it's crashed or not).
This boot is however, pretty iffy. I can get it to boot about 20% of the time on average but sometimes that's a couple of consecutive boots fine and then it might fail 10 times in a row, but I'd say a mean success of about 20%.
The way it fails is, as soon as it tries to load, if I see 'does printf work??' I know it's crashed and I need to hard reset. If instead of this I see '++++++++++++++'...etc I know it will boot fine. It seems I have a slightly better chance of a good boot after the system has been fully off for over 15 sec, but this is by no means always the case and doesn't guarantee a successful boot.
Hopefully this might help someone in the same predicament out in terms of actually getting High Sierra installed.
Regarding my further boot issues, should I just leave this on here or start another thread for that?
Thanks!
Rupert