If Melzer was just "some random opinions" it would be easy to dismiss. But as UN Special Rapporteur on Torture one should probably put some weight on his professional opinion. Or Swedish officials should explain why the allegations are wrong, but they reportedly refuse to do so.
Also if Melzer fabricates the allegations here, that would be a problem for credibility of the position of the UN Special Rapporteur on Torture and investigated. Another reason for Swedish and UK officials to answer in some form.
The article also contains original copies of the mail exchanges between the Swedish police (at least it says so), but I don't speak Swedish and so cannot read it.