Particularly for road bikes actually but since on-one are doing carbon mtb frames now – could be for MTB as well.

Low quality carbon can often be heavier then high end aluminium. Stiffness and vibration damping will be negligible usually, so all you're really getting is the ability to say "my bikes carbon".

Cheap carbon can be flexy and rubbish to ride, but then again so can expensive alloy.

the boardmans seem to review ok.
some mixed comments on the planet-x.
not read anything about ribbles or the evans pinnacle.

Price!

Ribble Sportive Carbon rider here – Love it 🙂 Had a Allez B4 which was fast but harsh 🙂 Just did 174 Km Sportive on it and was 142 out of 800 riders so that's my recommendation (sorry not bragging just showing you don't need boutique bikes to be up the front) 🙂

BTW: MudSux

I'd prefer a Trek 1.5 to a Claud Butler Torino. Plastic might be pretty but I dislike plastic frames.

none

Ribble do a carbon Evo model that can be specced for under £1000.
However, I don't know how they compare to say a Cannondale CAAD9 alloy frame who everybody seems to rave about.