Charlie,
As I’ve got your D2 sides and a set of the BGS ends I’ll try to check them against each other for height if that will be a help. I got the ends to convert the Mallard/blacksmith 6w composite to the BG version but the BGS ends and the Mallard coach sides are a mismatch, so the D2 will be the ideal use for them.
Have you made a drawing of the Culm Valley 3rd and would you be prepared to share it?
Duncan