The tau(-) --> mu(-)(ν) over bar (mu)nu(tau), branching ratio has been measured using data collected from 1990 to 1995 by the OPAL detector at the LEP collider. The resulting value of B(tau- --> mu(-)(ν) over bar mu(nu)mu(tau)) = 0.1734 +/- 0.0009(stat) +/- 0.0005(syst) has been used in conjunction with other OPAL measurements to test lepton universality, yielding the coupling constant ratios g(mu)/g(e) = 1.0005 +/- 0.0043 and g(tau)/g(e) = 1.0031 +/- 0.0047, in good agreement with the Standard Model prediction of unity, and also to determine a value for the Michel parameter eta = 0.004 +/- 0.036. This is subsequently used to find a model-dependent limit of the mass for the charged Higgs boson, m(H)+/- > 1.30 tan beta, in the Minimal Supersymmetric Standard Model framework.