how do i fix my speedfight 2 50cc seat lock?

I have a 54 plate (2004) Peugeot Speedfight 2 50cc bike and I have noticed that the seat lock only locks properly when there is a helmet under the seat (when there isn't , it remains loose). also, even once properly locked, I am able to open it again with random keys like my d lock key for example.

If anyone can help me with this, please do. I'm so stuck!

Thanks in advance for any help.

Shotty

p.s. I have noticed that the lock barrel has been dented. Could it have been broken into before and would it be best to just replace the complete lock.