"I was told it wasn't compulsory & now it is."
Are you sure? Ask to see a copy of the school's uniform policy. This will be a specific statement approved by the governors.
When I pressed my dd's school for the actual uniform policy (not just what the headteacher decided he'd like the children to wear, but the actual policy) they could not produce it. I strongly suspect the school did not even have a uniform policy! I would guess that most schools do have a proper policy, but it might not be the same as what you've been told.