Yes, I think this would be valid as an extenuating circumstance, depending on what you are setting out to prove. It would help explain why KS2 predictions are not ideal, or why any curriculum-related 11+ scores are lower than they might have been.
Although it might be worth a brief mention, I wouldn't overdo it in your particular case, however, lest it distract attention from one of your strongest arguments (the problem associated with a noticeable dip in your daughter's performance that occurred over an extended period either side of the 11+, and which the headteacher can confirm).
I think you have enough in the way of extenuating circumstances. Academic evidence is the key.
I think what I'm trying to get at here is that even though I may be armed with level 5s in evidence from school, if I can get further clarification that they are 5b's say, then I could argue that if the school had not been a failing one my daughters predictions would have been 5a's. Also that these circumstances may have affected her NVR and VR scores (but perhaps I am clutching at straws with this part of the argument).
Yes, I think you're right Etienne I need to just briefly mention it.