Lots and lots and lots and lots in the archives...
I'm very happy that there are people working this stuff out and getting their hands dirty fixing stuff, I do think it's a bit simplistic to state that it's down to any one thing though. There are many significant contributing factors to this problem I will start the list:
1. 'Cooking' the starter motor under the exhaust primaries - I have rebuilt the solenoid and cleaned up the pitted piston a couple of times to address this
2. MFRU relay not up to scratch - I have put another relay in-line to fix this ('relay mod')
3. Low tension wiring to solenoid - I ran a separate cable alongside the original one with a switch so that I could alternate between one and the other when the fault occurred so that I could prove to myself what was causing it. Each time the click happened, flicking the switch and trying again worked
4. Battery master switch - I replaced this with a new one to get it working again
5. New starter motor and solenoid
6. New battery - having not replaced the battery since I bought the car in 2003 (I think) I replaced the battery last year to fix the click problem
Each of these fixes have stopped the problem from happening (for a bit). This demonstrates to me that there are many different causes for the symptoms - all related to not enough juice getting to the starter motor itself. I think that all of the components in the chain degrade over time and replacing any one of them will normally put you back above the failure threshold. The only one I never got around to replacing was the HT cable running from the FIA key to the starter motor, that was the next fix I was keeping in reserve for the next time it started clicking, but I've sold the engine, loom and starter motor now!
C