Cannot accept calendar invitations on iPhone

Following the last iOS update I can no longer accept calendar invitations on my iPhone. I get an invitation by email which is correctly entered into my diary but there are no “accept, reject, maybe” options available (as there previously had been). When I view calendar on my Mac these options are available for the same calendar event. This is an issue others have had but as far as I can find there has been no solution offered other than a description of how it should (and previously did) work. Anybody know how to fix this iPhone specific issue?