I can't start my D1 V8, turn the key and nothing, I've checked the starter relay, getting power From the ignition switch, the battery feed and the shift inter lock? But the relay won't switch the solenoid wire unless I earth the shift interlock manually even with that done I get nothing from the starter motor.
I check the ignition wire on the starter motor and I'm getting power at ignition on (P2) and starting position?
But still no life in the starter motor.
Where could the ignition power be coming from?
I'm guessing the starter motor has had it, but I need to sort the wiring fault before replacing.

The day before not starting, I'd turn the key to start and get a clicking sound like a flat battery, hold it in start for a few more seconds and it'll fire into life.

Take the starter motor off & bench test it using a fully charged battery.
This will tell you if the problem is the starter motor or not otherwise you're just guessing.
Be careful as they can dance about a bit if all is working properly!

Ratae makes a fair point, however, you might first want to try undoing all the connections and cleaning them then putting them back making sure all are securely fixed. Make sure you disconnect the battery first! Also, while you're at it, check/clean the main earth.

I had similar problems as you describe with my RRC and that was down to the terminals on the starter needing a clean. Do make sure your battery is well charged too.

I've sorted it (kind of), I believe my neutral switch is knackered, I've earthed the terminal at the starter relay and it now works, added bonus of being able to start in drive lol