So after realising that my desktop PC has been running in BIOS mode (how 1970s and probably the result of multiple clones from HDD to 10k HDD, to 10k HDD in RAID0 to SSD and to another SSD) and with a free weekend I thought it was time to have a look at the MBR2GPT tool.
However in running the validate phase I was getting the error message:
Disk layout validation failed for disk 0
After following through a few red herrings on the internet I had a bit more of a dig into what the tool was up to and it appears that one of the first steps is to shrink the OS partition.
It appears that the fix is actually to preempt this and shrink the OS partition yourself (I reduced it by about 1GB – which made sense as one of the new partitions goes right in at the end of the disk), given these kinds of steps should only be performed by a person who knows what they are doing and understands the implications I won’t go into any detail as to how to do this other than providing the screenshot below.