check the engine to battery earth At work
At home

"I can't always believe facts I read on the web" - Charles Dickens

winner by default of the tractor vs caravan race

Agree with CG and others - check the engine earth, and the connections to the battery itself and to the starter.

Bear in mind that the starter is powered directly from the battery (switched via the solenoid) - this is necessary as the starter can draw the best part of a couple of hundred amps and therefore needs hefty cables to supply it. From your description the problem sounds likely to be with the starter itself (including the engagement mechanism), the solenoid (worn contacts, possibly?) or the starter motor wiring.

On the FL the button really just tells a computer to start the engine. The computer then checks all the other necessary parameters (valid key present, immobiliser off, clutch/brake pressed, glow plugs up to temp, etc) and only then does it energise the solenoid and thus cause the starter to operate. The fact that the starter is turning suggests that everything up to solenoid energisation is OK. I suppose there's an outside possibility that something is causing the solenoid to be cycled rapidly, but that's perhaps unlikely?

A pre engaged starter motor should not start to spin until it is engaged with the flywheel gear ring, thats the idea to reduce wear on the gear ring, not like the old type of inertia starter used to do.

Thanks to everyone for the advice, you all deserve a cold San Miguel. As soon as the weather cools down here I'll be getting the overalls out and checking the connections!

Gulp - happening more frequently so starter motor removed for inspection. Minor damage to pinion gear but worse, several missing/damaged teeth on the flywheel crown ring. Ho hum. Now over to a decent local German owned garage who have done more for me in a day than the Malaga Main Dealers did in 4 years. I'm off to raid the piggy bank and feel down the side of the sofa...

We feel your pain brother!  John
07 HSE Narvick Black
Land Rover Series One
Triumph Rocket 111 (sadly gone!)