It turned out that he had recently heard a drink-drive case in which the driver ran the laced drinks argument, and produced someone who claimed to have put several large Pernods in his pal's Guinness (risky - he might be charged himself). The driver said that he hadn't noticed the funny taste, but the bench didn't believe him. Having tasted the doctored drink, we agreed that it would be impossible to miss the adulteration.
For the record, I have pinched this from JWP Solicitors' website:-
The defendant must show that his drinks were laced, that he did not suspect his drinks had been laced and that had the lacing not taken place then he would not have been over the limit. In our experience a laced drinks argument is rarely successful without the lacer attending court to give evidence to confirm that the drinks were laced and it is usual for an expert biochemist to provide a report on behalf of the defence to confirm the mathematics of the defendant's argument.