Carry a soft hammer with you Hannes. You will need some help but have someone tap the piggy back starter solenoid whilst you turn the start key if it won't start normally. This will likely free the partially stuck solenoid armature which causes the intermittent problem.
Only a temporary cure of course until you can replace solenoid, but it will get you home.
My very low mileage 2013 Mini Cooper S suffered the same issues - it has stop start which was a real concern when the starter refused to play its part in busy traffic. Removed the starter ( has the solenoid built onto it) , dismantled the solenoid part, lubed it and popped it back on the car. It may have been a connection issue, I just don't know because its never played up since.
Oh thats likely tempted fate! Back to the 4/4 - I think I'd remove the starter connections, clean them up with emery paper and reconnect just be sure. With the battery disconnected!
- Worth checking the main fuse by the battery too - these can corrode and give issues as well.
