Hmm, a number of interesting viewpoints here but the reality is a little different. About 82% to 86% of codeine is excreted via urine within the first 24 hours of administration. The variation is due to genetic factors in the individual concerned.
Let's take the 86% figure to make it simple. About 40 to 70% of that is eliminated in the form of codeine, up to 16% in the form of morphine and the remainder in the form of a substance called norcodeine. However, it is possible in some circumstances that the amount of free morphine in the remaining 14% excreted in the 24-48 hour period can exceed that in the 0-24 hour period.
This is probably what has happened in this case. The codeine alone would have led to the fail but the morphine present would have been the icing on the cake.
Incidentally, the three strengths of Cocodamol prescribable in the UK are 8/500, 15/500 and 30/500 where the first figure refers to the codeine content and the second to the paracetamol content. The first is OTC Pharmacy Only whilst the others are POM.
Clearly as the OP mentioned the 30/500 strength, we can discount the 12.8 figure mentioned by kvalidir as this strength is only in proprietary pharmacy packs such as Solpadeine Plus, which are on sale to the pub