The stone is fairly easy to find, it's embedded into the south side of the bridge, just under the arch. When the Beckensall archive describes it a 80cm from the bridge, it's not a typo.
There's a convenient parking spot just on the bend of the road for those in cars, but beware the humpbacked bridge, it makes a blind summit, and has been the site of a few head-on dunches in the past, so beep as you approach. It's fairly easy to get down to the stone, there's a gate in the fence, and a permissive path.
If it's dry, you'd be hard pressed to make out the rings unless the light was very low. Fortunately, there's all this water right next to it, so the full motif is easy to moisten to bring out the detail.
I can't help but think that the people who put it here may not have noticed it, and that there may be others in the immediate vicinity. Given the bridges proximity to the carvings still in situ at Middleton and Shaftoe crags, and the portables at Old Deanham, it's a good chance there are more marked stones in the area waiting discovery.
Rumor has it there was once a large panel in the area, which has been covered over for a long time now. Hopefully it's still there and hasn't been broken up. Lots of the prehistoric stones around here have been broken and/or moved.