Dawes.. 700*25 Spesh Armadillos 110 PSI
Carlton 700*23 Vittoria Pro tetch 125 PSI
Wow--given that you're comparing rides on rough roads, I was surprised to see such skinny/firm tires. But since I've been riding 700x32 at 65/70 psi, we may just have very different preferences.
There's a very narrow window of acceptable fork offsets for a given head angle; so I suspect your Mercian has a slacker-than-normal head angle (racing bikes are normally 73 or 74 degrees head, with an offset of 43-45mm).
If you get too much offset the bars get floppy
Ewww, cycling debate!
Saddlebags at 10 paces?
Jan Heine at Bicycle Quarterly has done a lot of research into this, and concludes (in my incompetent summary) that longer fork offsets and normal-ish head angles make for more stable bikes, especially at low-medium speeds. His tests seem to be pretty convincing, if counter to common assumptions.
OK, I'll crawl back into my little poseur burrow now and leave this to the real cyclists....