  We report on the study of a sample of 25 field early-type galaxies, in the
redshift range z~0.1-0.5, selected on the basis of colours and morphology from
the HST-MDS. Field early-type galaxies define a tight Fundamental Plane (FP)
out to z~0.4, with scatter unchanged with respect to local samples, within the
observational errors. The intermediate redshift FP is offset with respect to
the FP of the Coma Cluster. The offset of the FP is found to increase with
redshift. The evolution of the FP is studied quantitatively with a
Bayesian-Montecarlo technique. By applying this technique, we find that the
offset of the intercept of the FP (\Delta \gamma) with respect to the local FP
increases as \Delta \gamma = \tau z with the following 68 per cent limits:
0.33<\tau<0.44 (for \Omega=1, \Omega_{\Lambda}=0) or 0.44<\tau<0.56 (for
\Omega=0.3,\Omega_{\Lambda}=0.7). In addition, we interpret the results in
terms of the evolution of the stellar populations, under the assumption of
passive evolution. In a single-burst scenario, the observed properties are
consistent with those of a stellar population formed at z>2 (for \Omega=1,
\Omega_{\Lambda}=0, H_0=50 \kms Mpc^{-1}) or 0.8<z<1.6 (for \Omega=0.3,
\Omega_{\Lambda}=0.7, H_0=65 \kms Mpc^{-1}). If a small fraction of the stellar
mass is formed in a secondary burst, the primary burst may have occurred at
higher z. Finally, the intercept and scatter of the FP found for field
early-type galaxies and for cluster data at z~0.4 are mutually consistent,
within the observational errors. If higher redshift (up to z=0.83) cluster data
are considered, the ages of the stellar populations of field early-type
galaxies inferred from a single-burst scenario are found to be marginally
smaller than the ages derived for the cluster galaxies. [shortened]
