Thanks to vorsprung, zigzag, and Graham for helpful replies. That Flyxi does look interesting, and cyclo-cross frames would be worth investigating, but I think both are a bit more than I need.
I'm simply thinking of replacing the frame on my PlanetX road bike. After about 7 yrs of use I begin to get anxious about wear and tear, esp. the striations under the fork crown, and around the B/B. Also, I'd just like a bit more clearance. 25mm tyres are the biggest the PX will take, but it'd be a nicer ride with 28, or maybe 30. Plan would be to swop all, or most, components to a new frame. No real need for 'guards on this bike, which is for fair weather, so I also doubt the value of complicating things with discs.