In reply to bozie:
I totally agree with Rich, when I rigged the bridge swing a couple of year's ago for the London Mountaineering club, there are bolts and guide lines on the opposite bridge and use 2x 60 metre ropes using tied off grigri's. I used an assitant to help lower using both grigri's at the same time.
Couple of points;
If unfamilar I would release a rucksac with some weight in it first just to ensure that the above method works and everyone is familar with the set up.
Secondly, because there is weight in the ropes I would have a sling that ties in the jumper to the bridge.
Thus;
Jumper ties in on slack ropes on the road side of the bridge and is checked off.
Slack taken up.
Jumper goes over the handrail and holds on.
Final slack taken from rope - should be to tension the ropes.
Jumper untied from handrail and is free to fall. The weight of the rope will pull the jumper of the bridge will stop any slips, trips or mishaps. Enjoy.