Q723: Do the police need a reason to stop me whilst I am driving?
No, the police do not need any reason to stop any person driving, attempting to drive or in charge of a motor vehicle on a road for a routine check. The police can then require that you provide your name, date of birth, driving licence and insurance details. Failure to comply with any of these requirements is an offence.
The police cannot stop a vehicle just to carry out a random breath test. They have to have a reasonable suspicion that the person has consumed alcohol or drugs. Once the vehicle has been stopped for, perhaps, a routine check, this can be ascertained through the smell of alcohol, slurred speech or glazed eyes. Failure to comply with a request to carry out a breath test is an offence and the penalty is the same as if you had been convicted of being over the limit.
The police can, however, breathalyse a person without a reasonable suspicion that the person has consumed alcohol/drugs, if they have committed a traffic offence whilst the vehicle is in motion (e.g. driving carelessly, having defective lights, failing to comply with a traffic sign [obviously not a parking sign] or using a mobile phone).