Sorry Susanb but what your dentist is saying is rubbish; what he means is it's not ecomically viable for him to be doing a root treatment for you when he could be doing more expensive things for other private patients.
I would question why you need it so soon after having a crown - if there was any doubt as to the health of the tooth, he should have done it during the original crown prep, I would have thought.
I would try to find a good nhs dentist if I was you. Word of mouth recommendations are best. (I know, they are rare, but there are some out there!)