If you feel heavy pain in your chest (and in some cases left arm) you should call 999 and ask for an ambulance.
Ensure the pain isn't temporary or a mild irritation such as heartburn and make sure you have alerted someone to help you if needed.
A potential heart attack is a medical emergency and should be seen to immediately. The operator will give you further instructions.
If someone can get you to Southampton General Hospital, dial 999 regardless so they can expect your arrival.