It is not unusual for a police officer to allow a detained person to drive away in such circumstances and it won't have any fatal effect on a challenge to the process. At the time, the police officer would not have had an evidential indication that the THC level was over. The roadside screening device is not definitive. It is also very plausible that the officer did not deem your son to be impaired at the time and took a decision to allow him to drive. Impairment is not a relevant factor for a charge of this type. However, best practice would certainly dictate that he should not have been allowed to drive home.
The incorrect date on the summons is not necessarily something that is fatal to the prosecution position. If this is an innocuous typo, the court would almost certainly give leave for that to be amended.
Further investigations would need to be carried out before any solid defence could be identified. You can contact a member of our specialist team on 0333 009 3334.
