Well I’m very glad to report that the upgrade worked perfectly.
I still don’t know why the PTHD10 wasn’t shown in my Avid account but when I entered the redemption code for the upgrade and support plan, it found the licence in my linked iLoK account and I now have a full upgraded licence for PT 10/11/12 & Ultimate + I/O and a support plan which it’s showing in my Avid account.
I would recommend anybody currently sitting with and old version PT (Especially HD) to grab one of the last few Upgrade & Support plans before they run out. I got mine for £550 from Thomann (I think they sell worldwide) also Sweetwater have them but want $999 which is a bit steep and there are a couple listed on Ebay
BTW I did purchase my PT 10HD from a legitimate business seller on Ebay (located in Canada I think) and looking back to my emails of 2012 he transferred the licence directly to my iLOK as Avid allowed that in those days. I thought I had registered it with them but that could be why it wasn’t shown in my account.