I discovered my issue.
At first I thought a full reinstall of win10 solved it, but it turns out the additional software for my Mobo was causing conflicts with the card.
My Mobo is an Asus ROG Z170 Maximus VIII Hero Skylake ATX and the card is EVGA GeForce GTX 980 Ti Superclocked+ ACX 2.0+
As far as I can tell, part of the AI Suite software, which can control all aspects of one's mobo from fan speed to clock causes issues with the gfx card and Home Designer, I did some more investigation and it also hit DAZ3d with a similar viewport issue.
Many other apps like 3ds Max, photoshop, games etc etc seem to be unaffected.
Anyhoo, deinstalling the AI suite seems to have solved the issue.