Thanks John.
You said "The equation is the usual TAS = EAS / (1/√σ) where σ is the density ratio ρ/ρo..." Yes. I understand. That's where I am at. I have the relevant pages in DCT Bennett's book "The Complete Air Navigator" This book was in my father's library. I would post these but when I tried just now, I lost my post. I think there is enough in the section on the calculation of TAS from IAS (page 170), together with Appendix IV (4.) on The International Standard Atmosphere, which has columns for 1/√σ and √σ, Temperature and Relative Pressure at height..
Jim