The S-matrix formalism gives an almost model independent description of
the Z lineshape which explicitly allows the $\gamma-{\mathrm Z}$ 
interference term to vary. Using LEP1 data alone the error on
the $\gamma-{\mathrm Z}$ interference is large and its contribution to the 
hadronic cross-section is strongly correlated with the Z mass.
By including the measurements of fermion-pair cross-sections and 
asymmetries at LEP2 the $\gamma-{\mathrm Z}$ interference can be 
determined much more precisely. Using both LEP1 and LEP2 data, the Z 
mass is measured in a 
model independent way with an error that is only slightly larger
than from a five parameter fit to LEP1 data where the $\gamma-{\mathrm Z}$
interference is fixed to the Standard Model prediction. This note is 
restricted to OPAL data, no measurements from other LEP collaborations 
or from lower energies are used.