I give up: JD 757 Starter won't engage

Hi everyone - my first post here. I've had an intermittent starting problem with my Deere 757 for a while and now it has become permanent. They symptom is that the starter just doesn't engage when the key is turned to "start" - no solenoid click, no nothing. Previously it would work every now and then but now: nada.

Things I've done/tried:

• Checked every wire in the starting interlock circuit - all good.
• Replaced all starter interrupt switches (brake, seat, control levers)...

I give up: JD 757 Starter won't engage