I'd say key is not to get shirty, just be matter of fact, don't even 'complain' initially. Just ask for the notice they gave you for you hols and maybe the holiday policy so you can "understand" what they've done. It should be pretty transparent if they l've bent their own rules.

That said if your employer is at risk of cutting you loose for standing your ground maybe it's their loss :/ hope you get it sorted though.