I'm not clear from your question whether you have been asking for an increase in your contracted hours, or whether you've been asking for overtime.
If you've been asking for an increase in your contracted hours, then it seems very unreasonable if a new employee has been recruited to do a similar job. However the company has not broken any law - it has just gained itself an extremely demotivated employee.
If you've been asking for overtime, then as Jenna says, every business I've ever been a manager in prefers to recruit full-time staff rather than rely on permanent overtime - it cost more. In one of my HR jobs, I instigated a policy that ensured that part-time employees, if they work overtime, only got normal time rates until they worked more than 37 hours in total in a week. The reason, I hope, is obvious - one cannot (fairly) have people on 37 hour contracts working alongside part-timers on say 20 hours, where the part-timer gets 1.5x pay from 21 hours per week upwards.
I trust that give you a view as to whether the company has 'done wrong.
You have a right to air your view on many things with your employer. If you are genuinely unhappy with a management discussion, most companies operate a process called 'grievance', which enables you to get a hearing with your manager, normally involving HR. But I am not sure you have a grievance here, unless you had an understanding that the availability of extra permanent contractual hours would at least be mentioned to you, if they become available, before recruitment from outside.