The waistcoat is from Simplicity pattern 2895 and was easy to make - I'm not 100% happy with the welt pockets, but I don't think anyone will be looking at them from a range of 6" so 95% happy will do. It fits well too! The only problem with it is the length, which I think is a little short - not surprising, since he has (in his own words) a freakishly long torso. Mental note to adjust for length when I make the frockcoat.
I was extremely please to discover not just 1 but 3 cutting diagrams for inverness coats while looking for ideas online.
Pattern 1
Patterns 2 and 3
At the moment I think I'm going to focus on pattern 1, but use patterns 2 and 3 for additional information as required.