Dentist ordered and

I agree with the replies from my colleagues posted above. Perhaps ask for an appointment or a convenient time for a phone call ,so that you and your dentist can discuss your concerns again.

Clearly explain again to the dentist in writing that you are unhappy with the crown and that you were not given the opportunity to check it before it was cemented (as would be normally done)
State that you do not wish to involve solicitors or the General Dental Council and you hope they will replace it as a good will gesture or give you a full refund
£300 is rather cheap for a private crown so you
May find you have to pay more at another practice but at that price the crown should be a reasonable match to the adjacent teeth

I know that when a dentist cements a restoration that a laboratory has made ,that will mean he takes sole responsibility for the restoration by fitting it as he or she has approved it. This cannot be your responsibility

I totally agree with both answers above.
It's the Dentist's responsibility to get your consent to cement the crown.
The tooth ought not to have decay in it now, but it is possible that leaving it alone will avoid further work.
Think twice before having it changed, is it very visible or is it near the back of the mouth ?