High speed vibration

[Saff]

OK, I have a problem with vibration at about 60 mph and above.I’ve had this since I bought the car about a month ago.
It feels like wheel imbalance but nothing can be felt through the steering wheel, more through the seat of the pants. It’s not too severe but irritating.
I had 2 new tyres on the rear when I bought the car and have had all four wheels balanced. I’ve had all the wheels off and scotchbrite the disc and wheel mating surfaces so are all clean and flat. I’ve rotated the hubs whilst observing the pads and nothing untoward there.
Wheels back on and evenly tightened and torqued. Test drove and a slight improvement but still there. I checked each wheel for runout with a makeshift dial gauge on the outer rim, all seem fine but cannot check inner rim. I guess the way forward is to get all the wheels professionally checked for true then onto a four-wheel alignment. I love the car but this is spoiling the experience.

Any advice and suggestions would be appreciated. Cheers Steve