Quick follow up on this for anyone interested - I got the Sargent seat recently and took it out for a test yesterday.
A quick 150 mile loop so I've not been on it for that many hours yet but it seems 100% better than the stock seat - (in my opinion).
It feels oddly flat and almost bench like when you first get on but just works.
On the stock seat I was getting a slightly numb arse and some hip ache after 50/60 miles - causing a lot of fidgeting and need to stretch my legs.
This has now completely disappeared.
In short, it doesn't feel like it will be comfortable - but it is.