Can't synch calendar to Outlook 2007 since upgrading to Windows10.

I can only synch calendar to Outlook 2007 when running iTunes as administrator. When running iTunes as my 'normal' user, the message 'Microsoft Outlook must be installed to synch calendars' is shown in the 'Info' window when the iPhone is plugged in. As Admin I see the 'Synch calendars with' radio button/ dropdown and can pick Outlook. There is no iTunes add-in in Outlook. This is since upgrading to Windows10.

How can I get the add-in, or otherwise fix this? I've tried uninstall/ reinstall of all Apple software on my pc (with anti-virus off, temporarily), and Windows 10 is fully up to date (as at 2 Nov 2015).

Windows10, iTunes 12.3.1.23, Outlook 2007 v12 SP3, iPhone 5, IOS 9.1