Blue Markers: No longer original. Today there is a replacement bridge, or ruined remains- or it is missing completely.
Maroon Markers : Intact original bridge dating to before 1750.
Red Markers : Intact original bridge dating to before 1600.
Ordnance Survey. Fully expandable to street level. Double click or roll mouse to expand. Or use the [+/-] in top left. Click/drag to move around. Click markers for bridge information. The red clusters open-up as the map is expanded.
For mobiles, suggest landscape view. Pinch/expand the [+/-] to enlarge the markers. Alphabetical List on the tabs is a useful alternative.
Note: If a bridge is not marked, then nothing could be found on any of the old maps.