@Heffalooomia "probable cause" is an American thing - it doesn't exist in the UK.
If they receive a call about a drink driver, the police will try to get behind the car, they'll see the driving and even if they are driving beautifully stop the car. If they suspect alcohol has been consumed (smell, behaviour, answers to questions), they will breathalyse. If the person blows a positive test at the side of the road - or refuses to do a test - they are arrested. If they blow positive again at the police station, they go to court. A borderline breath result might require a blood or urine test which takes time before the decision is made on court or not. If they blew negative at the side of the road, that's thr end of that.
At no time will the police disclose the name, address, phone number, details etc of an informant. Ever. It's protected information even at court. A judge could've insist on knowing the details for the selves, but this wouldn't disclosed to the defence.
Having been found drink driving by the police, how the police came to suspect the driver was over the limit is no longer relevant to the case.
If there's no one available or on the area to get to the car, it will go on the system as one to be watched on future.