What are the sources for Saxon and British/Welsh battles being at river fords

[Imperial Dave]

Yes Nick, the jury is still out on that one, on several aspects!

There is evidence that the dyke was built in sections over several centuries and not necessarily connected to each other. Also if you look at the 7th Century there is a Welsh/Mercian alliance and general cooperation. As you move into the 8th, there is a breakdown in that alliance.

Border marker, defence structure, coordinated or locallised, its a big mish-mash of viewpoints and possibilities...

In terms of the road battles question I am going to sensibly avoid it for now even though its in that neck of the woods! (oh yea gods I might have to go and have a look at it after all.... lol)

[Erpingham]

Offa's Dyke may (or may not) represent the terrain at the time it was built.  If you take either boundary or barrier explanations, the priority would be to place the Dyke where people passed the border.  You might leave areas where you couldn't take vehicles or herds (e.g. forest) till later.