This is a true packhorse bridge though all three were used by packhorses but not exclusively. Of the three, this looks the most original. Doubtless, all the bridges have been altered since the mid-17th century.
There are two buttresses on the northern side (other side ) formerly thought to be ‘cutwaters’; protection against high floodwater. After an expert checked, it was agreed these were to stabilise the bridge. Still, they proved an interesting puzzler.
The whole saga awaits the telling. It won’t be easy. Almost nothing, in detail, exists, no maps beyond the 19th century and the written record is very sparse. So, this challenge must wait for another time.
Just received this really interesting photo. In an issue of Cheshire Life, in July 1947, it shows the causeway leading to this bridge. 
Even better, it shows the causeway having a much greater depth than seen before, possibly ever! And, it shows water extending along the side of the causeway whereas today there is nothing but a vegetated mound.
What a cracker!