This sounds as if it is a drug test and not a
CDT test. When you say you relapsed a bit, did you do this just before the sample was taken? Also, did you inform your GP that you had relapsed?
They will check with your GP and ask them to go back 6 years to determine if you are an addict or an alcoholic.
Ask the DVLA to re-send the letter and then contact your GP and speak with them.
I am also surprised that the you need to sign a specific letter as for my 2 CDT tests I didn't do this prior to them contacting my GP.