Major disruption during a GCSE exam

If there are a number of exams for the subject, you might manage to make a case for discounting that exam entirely and basing the performance on the other exam(s) alone.

The exams officer would probably have to lobby the board pretty hard for it and it would need to have happened early in the exam ie affected the majority of her performance. I'd expect the EO to have to make a case for severe disruption (can't remember what the exact wording would be sorry) rather than just failure to implement arrangements.
Did she continue in the absence of the scribe? Did she wait long and therefore lose much time? How panicked was she during the exam? Did the scribe come back in, did they manage to continue? Can another teacher/invigilator attest to your DD's emotional state during the exam and/or to the disruption? That kind of thing, in a letter from the EO to the board.

Really they should have paused her exam until the scribe returned and then made sure she had the full remaining time. Do you know if they did this?

How long was she without a scribe? Did she get her full exam time (ie if scribe was away for 5 mins, did she get an extra 5 mins at the end)? You need to speak to the exams officer and then let him/her handle it - you can't do this yourself. Work checking in with them though, after you've made the initial enquiry - just to make sure they've done any relevant paperwork.

Special consideration will be given, but as you said the maximum allowed is 5%. I don't think they can do any more than this, failure to implement previously approved access arrangements is on the special consideration list so that's what it would come under.

Did the scribe come back? How distressing for your DD :(

What have school said? Their exams officer should talk to you. Or phone exam board direct.