Hi Tom - welcome to the wonderful world of long distance silliness
To be quite honest, it doesn't matter what you ride, as long as it works for you. Audaxers are quite idiosyncratic in their choice of machinery and you'll see pretty much every kind of bike on an audax, each with it's rider claiming that they have the best kind of bike for the job.
I have bikes from Roberts, Mercian and Thorn, as well as a cheaper generic frame from Raleigh. All have been great for Audax and all have taken me over vast distances in comfort.
It's comfort that's the key word here - doesn't matter what kind of bike you ride as long as it's comfortable. That's probably why all my bikes (and most of the bikes you'll see riders on) tend to be steel. But it's not just about what the bikes built with - it's about how well it fits!
Any of the big custom (and semi-custom like Hewitt) builders will make sure that your bike fits well. That's one of the things you pay for and it's worth it's weight in gold after the second century!