But my retainer kept breaking or I didn't wear it and if I get braces again it will cost me 700 ! Is that correct shouldn't I get it for free ? Through the NHS

Are you being treated by an NHS dentist ?

Hopp,
You said you had braces at 15, how old are you now? You also said your retainer kept breaking, did you keep bringing it in for repairs? Its important that the retainer is always worn, because it only takes a few days for your teeth to move. I am not sure if you stopped wearing your retainer if your ortho. Would be respondible for paying for your braces again.
(sorry, I don't know what HNS is, so I can't help you with that part)
I hope this helps you out.
Good luck.

If your retained kept breaking and you chose not to have it repaired then you will definitely be expected to pay to have braces put back on. It seems like the fee you have been quoted is quite reduced and this could be because you've had a relapse.

I'm not sure what is covered by NHS. Your age now may have something to do with why they are not covering your treatment.