I'm not going to worry about it too much, myself. The statute is quite clear that you can only be questioned to establish whether you appear to be a terrorist. The law has been broken in the case of David Miranda. I hope he will sue -- as he has threatened -- so that Special Branch know that they can't get away with breaking the law in this way.
Chapter and verse:
Schedule 7, 2 (1) An examining officer may question a person to whom this paragraph applies for the purpose of determining whether he appears to be a person falling within section 40(1)(b).
http://www.legislation.gov.uk/ukpga/2000/11/schedule/7
...
40 Terrorist: interpretation.
(1) In this Part “terrorist” means a person who— (a) has committed an offence under any of sections 11, 12, 15 to 18, 54 and 56 to 63, or (b) is or has been concerned in the commission, preparation or instigation of acts of terrorism.
http://www.legislation.gov.uk/ukpga/2000/11/section/40
If you're worried about this, perhaps you should also be worried about entering the US -- including in transit -- in case you are declared to be an unperson and sent to Gitmo with no trial or legal recourse. It's a bit worse than a maximum of nine hours of pointless tedium and handing over your Gmail/Facebook passwords which -- let's face it -- the spooks will have if they want them anyway.
As for Assange, if they get their hands on him, they will ship him to Sweden. He's spent the last year holed up in one room: I doubt another nine hours would bother him too much.