Thanks for the response Marty. Now solved
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The machines I had tried searching and general hunting in the Store for the App with are indeed both Win 8.1, that inclusive of the big update last Patch Tuesday. Had tried both 32 bit and 64 bit PCs and two different internet service providers all with the same outcome. Links direct to the App also failed.
However, knowing now that it is not an RT App raised the possibility that is may have been geo restricted in some way. I am in New Zealand and a search gives no result. However, if I change my PC's location to the USA Calibre Viewer comes up straight away in the search of the Store and installs fine.
I am away from home at the moment so will install it on a Calibre PC later in the day. On the face of it, from the Store page presentation, it looks very interesting and will give it a go - it may end up on my travelling notebook which I generally only use as read only with a copy from my master Calibre database on a spinning USB drive, so no editing needed and the desktop Calibre display being cramped on a 15 inch screen.
I'll come back in a few days with any comments.
John