On his presentment the accused was prohibited from raising the defence and he subsequently pled guilty. Famously, their Lordships [Lord Lane CJ at 33] held,
Possession does not depend upon the alleged possessor's powers of memory. Nor does possession come and go as memory revives or fails. If it were to do so, a man with a poor memory would be acquitted, he with a good memory would be convicted.
A contrary case that said that whether forgetfulness rises to the level of a defence should be left to the jury, R v Russell (1984) 81 Cr App R 31, was disapproved of. The Court held that had the court in Russell been aware of R v Buswell  1 WLR 64 the decision would have been different.
A similar argument was raised on an appeal in R v Tran  SASFC 85 recently. It failed, for similar reasons.
The accused claimed that he had placed the drugs in a cereal packet (where they were later discovered by police during the execution of a search warrant) when severely stoned a month earlier. He gave evidence that he had forgotten where he had put them, and this evidence wasn't challenged by the prosecution.
David J (Nyland and Anderson JJ agreeing, starting at 18):
... [C]ounsel for the appellant on appeal, in argument points out that in order to traffic in a controlled drug as in the present case, one of the basic ingredients is that he must have possession of the drug. He further argues that he must have actual possession at the time alleged by the prosecution. He then points out the well known dicta in He Kaw Teh v R (1985) 157 CLR 523 at 600, where Dawson J approved the definition of possession for the purposes of the criminal law given by Lord Diplock in DPP v Brooks, namely:
In the ordinary use of the word “possession”, one has in one’s possession whatever is, to one’s knowledge, physically in one’s custody or under one’s physical control.
[Counsel] argues that accepting the appellant’s evidence that he had forgotten where exactly he put the heroin he no longer had the heroin in his custody or under his control. He argues that therefore the jury should have been directed that to prove the appellant was in possession of the heroin that the prosecution had to prove beyond reasonable doubt that the appellant had not lost or misplaced the evidence when hiding it in the house as he said in evidence.
I do not accept that argument. The appellant’s evidence was that he had misplaced the heroin somewhere in the house, not at large. He had not forgotten that he had the heroin, he had simply forgotten where he had precisely put it within the house. In my view, he still knowingly had physical custody of it and intention to exercise control over it.
A number of cases were referred to by counsel. In Police v Kennedy (1998) 71 SASR 175 the respondent was charged with possession of child pornography contrary to s 33(3) of the Summary Offences Act 1953 (SA). The judgment dealt with the question of whether the respondent was actually in possession of the material which was alleged to constitute child pornography. The evidence was that he had forgotten of the existence of the material which he had bought in the mid 1970s and it was therefore argued that that did not constitute possession. Bleby J held that even though the respondent may have forgotten that he had the material in his possession, nevertheless possession had been proved. His Honour referred to authority in the United Kingdom that mere forgetfulness of the fact that a defendant has certain material objects in his custody or control will not be sufficient to defeat proof of the relevant state of knowledge that he possessed them.
[Counsel] argues that these cases are distinguishable because they involve a situation where an accused person had forgotten of the existence of the material that was required to be possessed as distinct from having misplaced it as in the present case.
I reject that argument. In fact, I am of the view that his evidence that he misplaced the heroin makes out a stronger case for possession than if he had forgotten about its existence. He brought the heroin into the house, he hid it in a cupboard, and at a later stage he had forgotten where exactly he had hidden it. It was in the house which he and his partner, the co-accused, occupied and it was consequently under his control. No-one other than the appellant and the co-accused, on his evidence, had access to the heroin.
The appeal failed and the appellant remained convicted.
Edit: Thanks to Habeas Corpus for the image recently attached to this post.