I do have a known good ignition switch you can have for the price of postage if I can get it out in one piece, I'll have a look at it tommorow if you like.
The inhibitor switches on C4s do fail, you can get new genuine from the states, same on all C4s I reckon.
Get a helper to crank the car while you check the voltage on the trigger wire at the starter. If thats good I reckon solenoid or cable issue.
If you dont have a good 12V at the trigger wire, go back to the plug for the inhibitor switch. The plug sits just behind the engine, has four wires. The two fat ones are for the starter trigger, one comes from the barrel and goes through the switch, the other one comes back from the switch. Bridge the terminals on the plug and see if that makes a difference, or just check whats there with the ignition switch in the start position. If that doesnt help, id go right back to the barrel and see if you're getting that 12V feed with the key turned.
Just a process of finding where the signal is lost in the loom, if thats the issue.
Good luck, pretty frustrating issue. Out of interest what sort of starter do you have on it?