Left: spectral index of the HT vs. distance from the “head”. The green line is the classical model of synchrotron ageing of a population of electrons that have been accelerated in the “head”. The injection spectrum is assumed to match the spectrum in the head of the tail. Right: radio surface brightness vs. distance; model normalisation (K_e in Appendix) is chosen to best illustrate the comparison with the data. A, B and C refer to Fig. 8. In both panels the model parameters are: ℳ = 1.7, θ_SO = π/ 2, a = 1, c_S = 1330 km s^-1, σ_v = 1400 km s^-1, ψ(t = 0) = 1, ηsinθ_O = 1, with θ_O = 60° and θ_SO = 90°, Γ = 5/3 (red line) and Γ = 4/3 (blue line).