[Graham Boak]

No, Deighton was just plain wrong, because the assumptions that went into his calculations were incorrect. The matter was studied by the University of Manchester, who published a report in the Royal Aero Society's Journal. I'm sorry, I can't quote the precise reference.

The usually-quoted parameters are
Sustained Turn Rate (or Radius) which is the maximum turn rate that can be acheived without loss of speed or height.
Attained Turn Rate (or Radius) which is an absolute maximum found at CLmax, but during which spped/altitiude is being lost dramatically.
Specific Excess Power, which is a measure of the maximum acceleration or climb rate at any given point in the envelope.

Most turning combats will descend in altitude and reduce in speed as each pilot tries to out-turn the other.

However, these are far from the only relevant matters. For example, Rate of Roll can be significant, and indeed transients such as the time taken to achieve that rate of roll. This was significant in 1941, where the Fw190 had such good ailerons that it could reach its maximum rate of roll sooner than the Spitfire. If caught at a disadvantage, it could roll and dive away, beng gone out of reach before the Spitfire could follow.

I'm sure that other examples, different in detail, can be found.