How do I unsubscribe to the calendar?

Delete the calendar it in Lightning, and restart Thunderbird.

What do I have to do to see "Google calendar" as an option?

Install this add-on. https://addons.mozilla.org/thunderbird/addon/provider-for-google-calendar/

Also see https://support.mozilla.org/kb/installing-addon-thunderbird

I notice that this version of TB is 32 bit and I'm running W7 64 bit.

That isn't a problem.

Thanks christ1. Using Provider, the two-way sync is working both ways with Google Calendar. I'm still not succeeding with setting Lightening up to sync with Outlook.com. When I do, the calendar I create in Lightening is unselected as well as being the coloured equivalent of greyed out in the side panel and cannot be selected. ie. The tick box remains unchecked even after selecting it by going into "Properties" for that calendar. Hope you can help here. (The clip just shows the relevant section of the navigation panel. I've removed the calender for Outlook.)