Hi Christopher, coming to this a bit late...
Are you sure your GG was SYH 103, there is a very comprehensive book on them which lists all of them and their histories. SYH 103 is all-but blank in that list meaning it had been sold out of service in the 1960s or 1970s which is why DVLA drew a blank for it. I know you had to end up registering it as 608 XVB. I wonder if the SYH plate was wrong in the first place? If you have the chassis number it will be easy to see if it is recorded in the book for a GG under a different reg.
You will need to look at the chassis number plate again though, I see from one of the check-a-number sites that it was registered as '108352' which is wrong, it ought to be RLHZ followed by a 4 or 5 digit number.
Martin
