This problem foxed some of you at first, but not for long! Elliot and Richard from Moorfield Junior School explain:
The lines down the middle of the symbols are mirror lines.
The right half of the symbol is a number or letter.
They sent us a picture of the missing symbol which they say is the number 6:
Ben from Emmanuel School and Emily who goes to Riccarton High School in
New Zealand, also wrote clear explanations for deciphering the code. All
three of these solutions agreed that the message said "Decode this".
Very well done.