What should be the gap when the contact breaker is open. The haynes manual i have only lists breaker less distributors.

I think it is a dizzy from a Capri, but was also on the P100 1.6 as i have just bought a new cap from Halfords.

Which type of dizzie --- Ford/Fomoco/Autolite or Bosch ?

The more common el cheapo Ford one usually has a black cap gap in old money is 0.023" to 0.027" -- initially set the gap on the wide end of the tolerance as the gap will close up a lot on the first few minutes running --- and remember to grease the cam.
Re-check the gap after a few miles.

Brown cap usually indicates Bosch dizzie I don't have the gap for that type to hand.

Its a black cap, is this the FoMoCo one?

Yes. If its a black cap it a Fomoco one, red/brown is bosch (however the side exit caps for the bosch ones are commonly black...). Its easy to tell as it has Motorcraft cast into the body just below the cap. Its also longer than the bosch one.