If a car has overheated, then it's standard procedure (and has been since I could drive in the 70s) to check that the head gasket has not been damaged by overheating.
There is a kit you can get - you have a liquid that changes colour when you out a special flask over the water cap in the engine if the gasket is gone.
That said, if the original cause of the overheating was a leaky rad, then that has to be replaced whatever. So if the garage have done that, they have fixed the cause of the problem. However for reasons best known to themselves (are you a woman, by any chance ?) they didn't bother to check for further damage, or inform you there was a chance the overheating could have caused further damage.
Were you given a written quote ?
In legal terms ... for the sums involved there's no much you can do beyond make your point in writing. It's bad service, but not necessarily actionable.
Also, blown head gaskets are the faults of dreams for shady dealers. A few bottles of radweld, problems appears fixed for a few weeks and then ... well nothing gets better.
Do they still have the car ? They might not release it if you don't pay. That could be fun.