Sorry, but I've asked for some reliable breathalysers, which are also useful... If I feel the need to gain more knowledge about the law, than I'm 99.99% sure THIS website will be the best place.
Edit
I was thinking what to get for someone's birthday, it's hard to think of anything for a person who has everything. Breathalyser seems like a fun and useful present, anyway to me it does...
It's a terrible gift, even not as a gift - It's a terrible item.
It will encourage someone to drink closer to the limit as they think they can self test and be okay. Wrong.
It's a road side SCREENING device. It's not accurate enough to be evidential in court so it's not accurate enough for you to risk your driving licence/life/someone else's life on.
A road side screening sample AND an evidential sample reading presumes that was the lowest reading at the given time. Often the amount actually in the system continues to increase after a sample has been taken. Self testing and getting a 20 reading could turn into a 35 shortly after.