The police officer at the roadside will administer a screening breath test using a digital breathalyser. This uses a "traffic light" system under which green indicates no alcohol present, amber some alcohol but below the legal limit, and red alcohol possibly above the legal limit. A refusal to provide a specimen at this point is an offence, but of itself does not lead to mandatory disqualification, particularly if you subsequently provide a specimen at the police station. This is partly to deter wasting police time.
If your reading is red, you will be arrested on suspicion of drink-driving and required to take a further test at a police station. You cannot be convicted purely on the evidence of the roadside breath test. If the police officer is in a car, he might take you directly to the police station, but more likely (and certainly if he is on a motorcycle) he will call for another police vehicle to attend the scene to provide transport.
At the police station, you will be required to provide two breath samples for the Intoximeter equipment, which is accurately calibrated and is used to provide the evidence of your BAC that is presented in court. The reading that will be used is the lower of the two samples. At this stage, a refusal to provide a specimen is an offence that is treated in law as the equivalent of being convicted with a BAC above the legal limit.
If your breath-alcohol level is between 40 and 49 µg, you will be offered the opportunity to take a blood or urine test as an alternative. This option should always be taken, as you have nothing to lose, and there is a chance it may result in a more favourable figure, particularly if you consider your alcohol level is falling. If the police fail to offer this alternative they have not applied the procedure correctly and this can be used as a defence in court. You also have the right to be given a sample of blood or urine for independent analysis, but experience suggests that the alcohol level is unlikely to vary significantly from the official test.
In practice, prosecution guidelines followed by the police mean that drivers are not normally prosecuted until they reach 40 µg of alcohol per 100 millilitres of breath, equivalent to a blood alcohol level of over 90 mg. However, you should be aware that the police will prosecute for blood and urine alcohol levels of 1 mg over the legal limit, if the breath-alcohol level was above 40 µg. This explains why press reports occasionally appear of individuals being prosecuted with a BAC of 81 mg, which seems to go against the discretion outlined above. It should also be remembered that the 40 µg threshold is only a discretionary one, and the police may prosecute for a 36 µg level if, for an example, a driver appeared to be very drunk, but gave an unexpectedly low reading.
To stop you from driving again while you are still over the limit, the police may keep you at the police station for some time. In some cases this can mean a night in the cells. Once you have been released from the police station you will be expected to make your own way home at your own expense, and also to retrieve your vehicle.
taken from:
http://www.80mg.org.uk/ddlaw.html
