Possibility of accuracy error in police breathalyzer test

I am contacting you on behalf of my 19 year old son who is a student at Bangor University, North Wales. Last Friday, he had one glass of wine with his dinner, after which he went out in his car with his girlfriend and flatmates. He was randomly stopped at 11.30pm by the police and breathalysed. The reading was 38 and he was taken to the Police Station. After about an hour at the station, he was breathalyzed again. This time the reading was 42 and another test one minute later, came out at 41. I understand there can be errors in the accuracy of the machines used and since we are talking a matter of one or two points here between him being convicted or let off, I am concerned that the fact he dropped one point in one minute shows there are some discrepancies. He has a clean record and uses his car for his part-time job. Is the possibility that there are inaccuracies in the machines something which would stand up in court if he were to plead not guilty? Many thanks for any help - I really don't want him to lose his licence and have to go through the associated insurance increases, and get himself a criminal record, if there is any chance we can fight this.
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