Eddy....
My initial at LGW was actually a "recurrent" because I already held a class 1 ICAO (Canada) However, I was issued my cert. on the same day by the Doc after the strip down and groin rub
With a little conjecture here, I assume that the urine samples are subject to basic checks and if anything else comes up, they may be sent off. Your best bet is to phone the CAA - it could in all likelihood be nothing, or maybe even that they were too busy on the day to check all the samples and issue there and then.
Good luck!