I was given

I was given root canal treatment in December 2013 and a crown fitted by the trainee dentist at my NHS practice. After about 5 months I had a major infection under the tooth and had to go to an emergency dentist who x-rayed it and said the root canal treatmnet had gobe into the gum and past the sack where it should be. I was given antibiotics which helped until I went back to my own NHS dentist. The trainee dentist then attempted to do the root canal treatment again and said she had managed to clean it out. She then did more root canal treatment but when the senior dentist checked the xray he told her to take it out all th e filler. I then asked for the senior dentist to deal with me and he said the they cannot get the filler out so if they fitted a crown again it would get infected again. He then extracted the tooth and said he will need to fit a bridge. I already paid £350 fro the first crown and they wanted another £200 fordoing the root canal treatment over again. He said he would only charge me cost price for the bridge which he say can be fitted in 6 months time. Is this correct or has the trainee dentist made a mistake and caused all this problem? What should I be charged for in this case?