Thanks for the replies.
I believe the Subaru dealer has checked the tyres and balancing and there is no steering/handling problem so it seems unlikely that these are contributing to the constant vibration when driving.
I have been told by Subaru UK that the power steering is now electric motor driven as opposed to the old hydraulic pump type so maybe the motor is causing vibration the steering column?
I worry that in order to save weight and reduce fuel costs it may be that some vibration damping material has been omitted from the new models, in which case I won't see an improvement with time.