"Agapanthers i am sorry but that is specious nonsense it is more or less impossible to download child porn 'by accident' or by 'searching for something for purely innocent and wholesome purposes' "
Really? I bet you could find child porn on Google/Bing Images in a matter of seconds using some innocuous search terms.
Why do you think they did this?
www.bbc.co.uk/news/uk-24980765
I don't think it's appropriate to comment on his innocence/guilt, but I would have thought that if you are charging with a number of offences, it would make sense to lay any possible charges at once, as they can be dropped at a later date.
That doesn't mean he has or hasn't downloaded anything of course, but this is not purely a case of downloading images. Where you have a man accused of abusing children, I would imagine that they would seize his computers as a matter of course and the computer would be scoured for anything that could possibly be deemed to be child pornography. Obviously since most people do not undergo this scrutiny, the vast majority of people that are legally 'in possession' of child pornography on their computers will never be caught.
"All the sites visited by someone on any given computer, and how frequently, will be traceable"
No, not necessarily. You can't make any such assertion. It is possible for the history to be wiped but still traces of images to be on the computer, and vice versa.
"so any accidental visiting or downloading can easily be ruled out by investigators."
Easily, no. If you visit say loydsbank.com (mis-spelling), then it might be owned by cybersquatters, and popup a pornography ad. That pornography would then be on your computer, but the source of it isn't easy to discover at all, because you would need to analyse the source code for the website that popped it up to determine whether you visited it deliberately or not.
"Prosecution would be extremely foolish to go ahead without being able to show the frequency of visits to sites where pornography allegedly popped up,"
Well no, they can lay charges when they find images that look like child pornography on his computer (whether they are in fact illegal, the fact that they appear to be is enough for charges). That's all they need to do. It's for Rolf's lawyers to defend that and perhaps get the charges dropped or at least plead not guilty if appropriate in court.
" and would also be foolish to go ahead without being able to show that images were saved."
Actually if you view a webpage, then the images/text are saved on your computer as part of the process of viewing the page. They will be there without you taking any action at all, and may also be recoverable even if you empty your internet files. In terms of 'possession', then simply viewing a web page would be sufficient. It's not necessary at all to show they were saved.