As Lyn mentioned previously knowing his current testosterone level is the best place to start. If his testosterone is castrate level then the Decapeptyl has worked and pursuing the dose size is irrelevant. In that case it is unfortunate that his cancer is now castrate resistant, but other combinations of drugs may help for many years.
If his testosterone is above castrate level (I'm hoping Lyn will post with what castrate level is as a number) then either he was underdosed or his testosterone (but not his cancer) is resistant to Decapeptyl, and another HT will probably work. To get any further we need to know if he had the correct dose.
You have done remarkably well to get as far as a batch number and managing to find an error in the logging at the GP practice. Remember we are not trying to get anyone in trouble, we are trying to ensure they have rigorous procedures in place to prevent errors, allow the tracking of errors and hence save lives. You won't know how many lives your actions will save by highlighting this small error at the GP practice, but hopefully from this day on everything will be recorded correctly and thanks to your actions lives will be saved in the future. (How strange that in ten or twenty years time your actions today may save a young child's life, of course s/he won't thank you they will just assume the correct logging of drug doses is the norm, as it should be.)
Now to more pressing problems how to tie the dose to the batch number. Well before we engage in battle we have to know our cause is just. You won't be surprised after my previous paragraph that I am in no doubt our cause is just, but if you are in any doubt ask yourself these questions: With what little information we have can we be sure that batch of drugs was safe? If we knew the dose we would be in a better position to judge if the batch was safe. Can we stand by not knowing if people may die because of a poor batch? It is our duty to tie that batch number to the dose size, not for your grandad, but for all mankind.
How do we fight bureaucrats? First we must understand what motivates them to fight us? Laziness. That woman was probably happily polishing her nails until you had the audacity to phone her.
The next thing we have to know is that all human endeavours involve a hierarchy, and everyone in that hierarchy is lazy and wants to pass the work either up or down. Passing it down is easy unless you are at the bottom; passing it up is not so easy because the person the next level up is equally lazy, s/he will try and refuse to accept it, if they do accept it they will either have to do the work themselves (heaven forbid) or pass it up.
So that woman on the phone has a nice reply, "that's a reportable error, and I'm too low in the hierarchy for that, now what sort of nail polish goes with these shoes". Now as you have said it is ridiculous to report this, and the moment you do that, someone higher up the hierarchy is going to find they are having to work on something ridiculous, passing patently ridiculous work up the tree is impossible. So now the easiest way to avoid hard work is to just give you the answer you want. So go ahead follow the bureaucratic process, you have nothing to lose but the people who are being obstructive have, so they will give in.
And remember you are doing this to improve the drug companies procedures, so that many more lives may be saved in the future.