The Tamar forms the majority of the border between Cornwall and England, and I did all the bridges (or at least most of them) some years ago before I had GPS or thought about Google satellite images and online OS mapping, so decided to do it again, but properly this time.
Numbering the crossings from the north, I did 1 to 14 inclusive yesterday.
Drove up to a big layby just south of Kilkhampton and parked up, then set off for a 40mile ride. I've just about completed riding all the roads in Cornwall, so I took in a few of the last ones as well.
Here's a map of my route with the crossings numbered, and a complete list too.
I tried to get to 28 Bude Canal Aqueduct but there's big signs up forbidding cycling and horse riding, plus there were walkers about so didn't want to flout the rules. When I realised it was a aqueduct, I aborted my walk. Initially, I thought it was a normal bridge but found a plaque telling all about the Bude Canal ............ aqueducts and railway bridges don't count for a cyclist wanting to ride over them!

When I've done all 27, I may drive up to the canal and walk it with Doggie. Meanwhile, Calstock Viaduct is easily done on the Gunnislake train, and it's only a matter of getting a return to Saltash to cross the Royal Albert Bridge.
Tamar Suspension Bridge (27) will have to wait until next year as the cycle/pedestrian part is closed and not do to be finished until April (I understand).
Next outing will be driving to Launceston to take in 15 - 21 for another 40mile ride.