WiredPrairie hit the issue head on.
Server chap had some new links in the page with target="_black" which forces a new tab in the browser. So is assume the Windows Store App webview does not allow for tabs and it forced the web page to open the IE browser on the device.
Once removed, issue gone.
Was not an issue with the iPhone/iPad or Android version of the app and not sure why. Same back end code, either way fixed.