Root Canal Treatment in UK ???What would you like to ask?

I am an NHS dental patient, in the UK yr 2011. I have just had an abscess under a crowned tooth which has previously had root canal treatment just the once. My dentist infromed me that my options now are to have the tooth extracted or pay �700 privately to them to carry out a second root canal. I dont want to lose my tooth and dont see why I should have to just because I dont have �700 spare cash to pay for this. I also dont understand why they are saying I have to have this done privately at a cost of �700 when I am NHS Exempt and should get this done on the NHS. I wouldnt mind if they would pay the NHS price for root canal of which I am entitled to and me pay the difference of which I would be more willing. I dont know who to contact about this or how to find out if they can do this or not ????