Appointments+ and Google calendar integration - Fatal error: Class 'Google_IO_Curl' not fo

Appointments+ and integration with Google Calendar has worked fine.
However, recently I get the following error message on going to
- the Calendar page
- at times on updating the Appointment Plus settings
- in the Appointments Plus admin settings, clicking on the Google Calendar tab and clicking on the Test Connection link:

Fatal error: Class 'Google_IO_Curl' not found in /clientdata/apache-www/c/h/chooseyourpath.com.au/www/wp-content/plugins/appointments/includes/external/google/IO/Curl.php on line 177

Also, the calendar shows the available times fine but when a client goes to book a time slot, when saving the appointment they just see the spinning wheel. On clicking 'cancel' the appointment is made and user receives their confirmation email and appointment is made in the system.

Any thoughts greatly appreciated.

I am using A+ version 1.5.1 on WordPress 4.3.1
Many thanks.