We report experimental data on the characterisation of two molten polyethylenes. The linear and non-linear viscoelastic response is determined for simple shearing flows using a standard mechanical spectrometer. The linear oscillatory data are modelled in terms of a discrete spectrum of Maxwell elements and the non-linear theology in terms of a K-BKZ type Wagner damping function. Self consistency for simple shear rheometric deformations is then established. Experiments are carried out for a centreline planar extension flow into a slit. Velocity kinematics was obtained using laser velocimetry and the stress profile from flow birefringence. The constitutive damping coefficient determined from simple shearing flow was then tested for the case of planar extension. A reasonable agreement is established between prediction and experimental results for one grade of polyethylene in planar extension; however the second polymer shows a larger strain hardening response than would be expected from the constitutive model used. We conclude that in the case of one polyethylene, the extensional and simple shear responses are intrinsically different.