driving

Apologies. The main question is, when is it legal to drive post treatment

Yes it is legal.  However, I believe the law is written such that if you are taking opioids and either feel impaired or are found to be impaired (especially if you have an accident) then you have broken the law and face the consequences.  If you look at the information sheet that came with your drugs then the relevant information should be on it. 

Just remember that the body metabolises the drugs relatively quickly so if you only need them to help sleep overnight and take them before bed then it has probably all gone by the morning - providing you've not taken any more.

Speak to your GP...if he/she gives you the OK then there should be no problems.