If you’ve got anthing written down sounds like they owe you the cost of radiator in the hallway (did they quote for 2 originally and then decide that one downstairs would be sufficient?) plus piping, fittings and labour.
Will your house be warm enough without this radiator?
What else did the job incldue? Was it a new boiler, all new pipework and all new rads? Or a new boiler, new rads using existing pipework and a powerflush?
Finally, what sort of size is the house? If its all new pipework in a very large house then the cost of copper piping might be worth considering.
IMO if they’ve done a complete refit (boiler, piping, rads and connectivity to domestic hot water, earthed all the piping) in a modestly sized house and doen a tidy job, I think that their price is not unreasonable.