Wow, thank you for all your replies.
The line is being driven by a 1 to 4 balun, with the signal generator being unbalanced.
Forgive my ignorance by if the impedance was changing significantly with frequency we would see more ringing at higher frequencies? We don't see anything like that, the frequency is relatively smooth but with higher attenuation at the upper end.
1 to 20MHz is used for the test because it represents the maximum data transmission rate used by the final application. Our products are used in real time seismic systems which incorporate thousands of multiplexed channels - the bandwidth requirement is massive :).
What I will try today is testing a cat5 twisted pair (if I understand correctly) on our instrumentation and post some screenshots of the results.
I am going to make a very basic testing system independant of our automated equipment and perhaps perform a comparison.
I will report back with my findings, thank you all very much for your feedback and comments :)