I think that because The Director of Public Prosecutions has said that they would not consider trying Brady for the two 'unsolved' murders, there are no grounds legally to obtain the five slides.
His reasons were the cost involved plus the fact that Brady is never going to be released anyway.

Brady himself has asked never to be considered for release, and I think he may have gone on hunger strike before, but the court of Human Rights insisted on him being force fed, bit hazy on that though.

Maybe there is a time scale issue too, I don't know if there is any legality barring a prosecution due to the time lapsed between the murders and now.

But the fact remains that those five slides are not technically evidence, as there is no prosecution and unlikely to be one brought.

Hopefully Bennetts brother will outlive Brady, and then discover his brothers grave once the slides are examined, so as to provide a proper burial. A sad tale.