The buyer's solicitor will usually send the vendor a questionaire that is much more detailed than the PIQ, and usually does ask about notifiable electrical work. Certainly this was the case when we sold my mother's house in 2007.
The PIQ became a requirement to add to the Home Information Pack in April 2009, so it's not surprising it wasn't used in 2007.
I would not expect solicitors to add another similar questionaire to the PIQ. Even if they do, the purchaser's solicitors would advise the answer "Not known" in all cases where there may be some doubt. This is because a buyer could pull out because of a wrong answer and that could initiate a claim for aborted solicitors' costs.
Virtually no member of the public knows if the work they've had done is notifiable and since it's the electrician who notifies, they can't know. So even if the question's there it's not going to get an answer.