Mmmm. I have a judder too at 50-60 ish, and was putting it down to an alignment issue as I've had it back to the garage a couple of times. I poly bushed my wishbones last year, but wonder are they now responsible ......
Must check the bolts 
Bushes in the omega actually form part of the suspension set up. They contribute to the ride quality as well as the main purpose of controlling the wheel
position. It's a trade off manufacturers deal with.
Too firm a bush and the car will be accurate but very harsh for occupants. To soft and the ride will be comfy but a nightmare for the driver to control the car.
What we also noticed in fitting poly was that the firmer bush material transmitted wheel imbalance and break judder to the driver that was previously absorbed by the oe bushes.
It's not that the bushes are the cause, they can't cause judder. It's just that the connection between the driver and road wheels is more direct with poly. So you will feel faults that previously went un noticed.
Which is a good job IMO, as if there's a fault, I want to know about it personally. Especially with brake judder, as this is often caused by the wear line that develops on the outer edge of the discs inner face.

Poly can also highlight the fact most tyre fitters can't balance wheels to save thier lives.