@ Christoph
And that is the beauty of a strict liability offence - typically there is no defence possible and you are assumed guilty merely for possesion, no arguing about how it came about.
The CPS still have to take a decision on whether or not it is in the pulic interest to proceed, and no civil servant is going to put themselves in the position of saying the defendant had CP, but I was OK with their explanation.
In effect the whole process grinds on through the motions and one more name is added to the register (sorry!), some people will complain about the courts not listening when the real complaints should be directed at the government which passed such nasty legislation.
I fully expect to hear a spurious argument back around how can you possibly defend those who have this material on their HDD, so consider this scenario:
Someone who isn't as technically minded/paranoid as the readership on here gets misdirected to a page infected in the manner described. They see something they find distasteful and immediately backtrack, forgetting about it after a few days.
At some point in the future they decide to take the machine to the local PC shop to get something fixed/upgraded - technician finds the filth, plod are summoned and another life is ruined, because the law says possesion = liability.