If you are a First Aid situation and you think you need a defibrillator, call 999 immediately. Emergency services will tell you where the nearest defibrillator (AED) is situated and how to access it. They will instruct you to stay and administer CPR to the patient while someone else goes for the AED. If you do not have someone with you, stay with the patient and wait for an ambulance or First Responder to bring an AED. 

A defibrillator is very user-friendly and as soon as you turn it on, it will give you instructions on what to do. 

For more information and an instruction video, visit the St John's Ambulance information page here.