It is possible to make an online appointment for existing patients. Click on the lurid pink vertical "Make an appointment" tab my ears are pointing to (very left of the screen). Apps can be found by searching for "appointuit" in iTunes/google play store.
Please note that option 1 or 2 "With regular practitioner" or "Select a practitioner of your choice" gives a selection of appointment times whereas "next available with any practitioner" gives one appointment time (that may be in 5 minutes time!). Option 3 gives the full gamut of times including 6 weeks into the future.