£150 for a filling. You were robbed. I have a private dentist, but a filling costs me £35.00. My dentist doesn't do anything on the NHS, but I knew that when I signed up. That's expensive. Go to another dentist. That's the problem if you have a dentist that does both I suppose. They will always try to sell you the more expensive thing.
A single private filling is cheaper that a filling as a Band 2, NHS treatment - at least it is at my SW19 dentist - and that is what the dentist recommended some time ago.