I think I might have found my way forward with the canal and how to cross it by road. This is obviously in the very early stages and is, quite frankly, a pigging mess. At the same time I have settled on the riverside fence to line the inside of the canal. The canal side roads will be adjusted once the bridge is in place as I know trying to pass roads under other roads or bridges causes real routing problems.
I must admit to spending a very frustrating time yesterday playing around with retaining walls. In the end it was easier to use the fence. As for colour of the stonework it is not so out of place as many buildings in that area of Wiltshire are made from Bath stone which is a similar colour. Given the price of it, even in Georgian times I doubt very much it would have been considered for the canal. Maybe where the canal meets the river Avon in Bath. I will need to check.
https://i.imgur.com/FOyqhpa.png