what makes you think you have been overcharged?
Are you asking whether they would get this sum if you went to court?
It depends what he is done and what the value of the job is.
If you didn't agree rates in advance then he can produce evidence of the average rate for that type of work.
Actually though, for three days work, £450 is not likely to be accepted. These are qualified specialist tradesmen. Even labourers earn more than that.
To be wholly honest, if they went to court they would probably get the full amount and costs and interest. The sum in question is not a case of obvious overcharging. It might be a little on the expensive side but no more than that.
Sorry if that is bad news.
Can I clarify anything for you?
I'm not sure what your further question is?
Put simply they are entitled to be paid the value of the job.
Ok. Good luck then.