All sounds a little murky to me, minty. Is your dentist suggesting that you cant have a procedure on the NHS because it will take up too much of their time? NHS dentists are reimbursed by procedure, not the amount of time that procedure takes. If this is what they mean, then I think they would be in breach of the NHS dentistry guidelines.
I would definitely get some clarification, preferably written, from them, detailing exactly what work they think you need and exactly why it would not be suitable under the NHS charging system.
You might already have a copy, but I have included a link with 2009 NHS dentistry guidelines, procedures, charging bands, etc.
http://www.aff.org.uk/linkedfiles/aff/health_a dditional/guide_to_nhs_dental_services_in_engl and_2009.pdf