I think I would tell the panel, as calmly as possible, about the head's phone call.
Some panels might be sympathetic if they thought you had been unfairly treated. The problem, of course, is that no one can prove exactly what was said. There's a world of difference between the head offering a place, and saying "We usually have some unfilled vacancies, and if you put your name on the waiting list, you will be eligible / could well be eligible / might be eligible ,,,,,,,,"
It's not clear to me whether this is a foundation school handling its own appeals, or whether it will be a local authority panel.
If the head is not in attendance, I would be tempted respectfully to ask for an adjournment and to request the panel to seek confirmation of exactly what was said. The head will either confirm your version of the conversation [which could strengthen your position], or he won't .........
Assuming there has been no maladministration with regard to the admission arrangements, then the test to be applied is whether your reasons for requiring a place outweigh the prejudice to the school.
If you haven't already done so, then I would suggest reading section C of the Q&As (C2 deals with applying to an oversubscribed school).
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