Just had time to check the new conversion, and it worked using the latest Virtualbox command I pasted, copied it directly to the repository, and it was recognised and booted ok after attaching to a VM.
Many thanks for the help and apologies for the hassle! 
@planedrop Yes, I'm trying to import using Xen Orchestra. I tried importing from the command line after copying the vhd manually to a storage repository, but I can't get over the invalid uuid error 
I tried running uuidgen from my Linux machine and also on a XCP-ng server just in case there was something different. From what I've seen, it should be just a "uuidgen -r" to get a compatible uuid.
Maybe I'm missing something really stupid and basic. I will try to go through the Lawrence Systems videos and find that one maybe there is something that helps. Today I'm expecting have work outside for most of the day I think, so there might be a delay in my replies.
Thanks