Hmm, maybe I'm wrong. We were told there were bustitutions from Cambridge all weekends between now and end of October, but planner on national rail enquiries site seems to think there are trains from KX to Cambridge on Sunday after all. Looks like it's just Saturday affected this weekend.

See http://nationalrail.co.uk/service_bulletins/currentAndFuture.html if you want to try and make more sense of it. You'll have to search for Cambridge to find the relevant sections.