Disco 4 (LR4) Steering wheel vibration

Got a 14 plate Disco 4, on 20inch/michelins - drives fine on road but getting a very slight vibration off road, rough track, stones giving a vibration feeling through the steering wheel.

Its on poly bushes but dealer says zero play on vibration plate, so have been changed in the past - I'm convinced it's the poly that causing it as very similar to worn rubber bushes i've had on my L322 but not as bad. Don't want to swap them out unnecessarily ...

What else could be it be?

If you've got poly bushes, then you will get more feedback from the wheels through the steering wheel.
The D3/4 suspension bushes are designed to absorb the bumps, so they don't come through the steering so bad. However removing the compliance of the standard bushes, means that movement has to go somewhere, so it's transferred to the track rods instead. This results in the surface being felt through the steering, as well as accelerated track rod and rack wear.