2015 A4 Allroad 2.0tdi Vibration at speed similar to wheel balance?

Hi
Just had 4 new tyres fitted about 6 weeks ago, they are Avon btw, but from day 1 I've noticed a vibration at speed, approx 50-60-70 mphs, that seemed like wheel balancing.
I returned to the tyre garage the next week and they quickly re-balanced all four wheels, promising them to be near perfect now and were not far out before hand, but the vibration remains???
They checked the tracking and found it to be spot on and I don't have any pulling or vibrating through the wheel.
It doesn't change during braking, and nothing is felt through the pedal, so warped discs are not suspected.
The vibration seems speed related rather than revs, same speed in a lower gear doesn't change it.

Any ideas?

Cheers
Mow