User482 said:
The path is a b*gger to find! If you're heading out from Bristol, you need to turn left off the dual-carriageway from Avonmouth, just before the main road junction. Easy to miss.
That takes you onto the downstream path.
To get to the upstream (Gloucester side) path, carry on a bit to the main roundabout below the motorway, and make like you are heading to the services. The cycletrack is on your left immediately you leave the roundabout.
The bridge over the tollbooths connects the downstream path with the services, between the petrol and the conference centre place. There are steps at the services end, and no ready access to the upstream path.
If you use the downstream path, you have to duck underneath the motorway at the Chepstow end. You can then either continue into the Bulwark housing estate, or climb back to the upstream path.
The upstream path continues along the NE edge of the A466 as far as the A466/A48 roundabout. There is access into the Bulwark estate at several points.