The clutch arrangement needs to be able to go on the splines of the Ford gearbox while being small enough to fit inside a clutch cover that fits on the flywheel.  There are a couple of options here, the most expensive one being the Frontline or Caterham clutch kit.  The second option involves using a Ford cover plate and cover, the third uses the Rover cover and a suitable clutch plate.

The clutch plate has to have a centre spline arrangement that is 1 inch wide and has 23 splines in order to fit on the input shaft of the gearbox.  The original Rover clutch is a 190mm one, so if the Rover cover is to be used then the plate needs to be less than 190mm.  The one I’ve used is a Hillman avenger clutch; this fits inside the Rover cover, and was also fitted to these cars.  Note that if a 1.8 engine is used, the smaller flywheel from the 1.4 or 1.6 will be needed to fit inside the bellhousing.

Car Mark Engine size Year made from Year made to
Capri mk2 1.6 01/1974 02/1978
Cortina mk3 1.6 09/1970 08/1976
Cortina mk4 1.6 08/1976 08/1979
Cortina mk5 1.6 09/1979 08/1982
Sierra 1.6 10/1982 05/1985

The clutch cover from some Fords apparently fits the Rover flywheel without modification, although some say that the flywheel has to be re-drilled.  Frontline retail a 200mm clutch kit which they recommend for the higher power K series engines.   Here’s mine:

I’ve used the Ford clutch arm coupled with the Rover cable and Ford release bearing for actuation.  The Ford arm had to be modified to fit inside the Caterham bellhousing: chopped the pivot end off a bit to make it sit nicely.  This meant a spring had to be joined to the back of the arm to stop it falling off inside the bellhousing:

This also pulls the release bearing off the clutch plate when the clutch isn’t in use.

The cable goes out through the hole for it in the bellhousing (secured by a bit of brass plumbing) and to a bracket mounted on the inner wing.

Clutch is light and bites low, the Rover cable is self adjusting so may work as intended.

Edit:  June 2012 the engine came out to sort out a starter motor problem, and that Hillman Avenger clutch I was so pleased about had exploded somewhat and half of it was missing.  The car now has a Quinton Hazell C784AF plate from Redline Car Parts, which has been fine so far (Jan 2017)