What sort of clicking? One click as you hold the key in the ‘start’ position means the solenoid has fired but not the starter. That probably means that the starter isn’t getting enough current – think discharged/faulty battery, poor connections, burnt-out contact on either solenoid or motor.
First port of call would be charging the battery.
If that doesn’t help, next time it happens, stick a screwdriver in to bridge the two terminals on the solenoid (no idea how practical this is on a focus in terms of access). If it spins up then, the problem is with the contact in the solenoid. If not, you’re looking at duff connections elsewhere or a duff starter.